SOS........CPP disc conversion kicking my butt

[FLYNAVY53]

Ive spent the last 7 months rebuilding a 1960 F100 for the family of a buddy who unfortunately died in a helicopter crash a few years back. The truck is 90% complete but the brakes are kicking my butt.

I used the CPP front and rear disc conversion kit with the power master and proportioning valve. All brake lines are new and the flex lines are braided stainless. The problem, as it sits, is that while there is SOME braking force, the pedal is complete mush.

Per CPPs directions and technical support, I've:

Bench bled the master, ensuring its placed level, in a vice, and the piston is depressed slowly and repeatedly until there is absolutely no bubbles present.

Adjusted rear integrated parking brake to within the CPP spec of 1/8-1/16 inch movement of the lever arm.

Installed the bench bled master and bled the entire system using a Motive power bleeder. In this process, the rear calipers were removed from their mounting brackets and positioned to ensure that the bleeder screw at each caliper was at the highest point during bleeding. After pushing over a gallon of fluid through the system, I am confident there is no air remaining in the system.

Currently, after you bleed the system, the pedal is hard and firm. As soon as you fire the truck up, and the vacuum booster fills, the pedal becomes mush. When driving, there is enough "bite" to safely drive around the neighborhood at 25-30 mph, but the pedal is mush for the top 2/3 of the pedal travel, and from a 30 mph roll, you can't lock the brakes up at all.

What confuses me is that I have almost the exact same kit on my '72 Suburban (minus the integrated parking brake on the rear calipers), and have never had any issues. Driving the two trucks back to back is a night and day difference.

Being at a loss, I disconnected the master from the proportioning valve and plugged both outlets. I then checked the pedal firmness just to verify there wasn't any internal problem with the master cylinder, which there wasn't.

So at this point, I am officially, 100% out of ideas, and 100% frustrated. Any assistance would be greatly appreciated as Im doing my best not to turn sour on this project in the 11th hour.

Thanks guys!

[ilikeike]

I'm just now chasing a soft pedal with the CPP cadillac style rear brake calipers,

I'm sure this is where my problem is, since My wilwood master,proportioning valve and 6 piston front calipers were fine before. I've done the E-brake adjustment multiple times, removed the calipers pointed the bleed valve straight up to remove any air..........the only thing I haven't done is to plug the rear lines to isolate them and see if I have a pedal.
I'm thinking it was a mistake to put this new rear on with these brakes and not just buy some wilwoods.

[FLYNAVY53]

yea....Im with you....my Corvette is going full Wilwood (if I ever get to it), but this being a mostly stock rebuild didn't really justify anything other than a basic disc upgrade. The thing I don't quite get is, even if the rear calipers with the E-brake are SLIGHTLY out of adjustment, would that really affect pedal feel this much? I mean my pedal is a straight up limp noodle. And after re-addressing the e-brake adjustment multiple times, the "bite" has gotten better, but the pedal still feels like crap.

I attempted plugging the rear line to isolate it today, but thats harder than it sounds given the way they have the proportioning valve and hard lines set up....I tried for about 30 min. but couldn't actually get the rear out of the master with the front still connected without significantly bending the front hard line. Let me know if you have any success with it.

Like I said, I used the rear calipers minus the e-brake on my suburban and had zero issues.

[bryant]

i ran into this with a cpp kit. did all the above with no improvement. finally i got some brake pressure gauges and would only get about 700psi. once i told cpp that, they asked what the engine vacuum was (14in/hg). they said thats not enough. it needs to be at least 19 in/hg at idle for their booster to work. we swichted to hydraboost and all of a sudden the brakes worked great.
so check your engine vacuum and if its not over 19"s then you have found your problem.

[FLYNAVY53]

thanks for the tip....Ill look into it tomorrow

[Rod]

The thing I don't quite get is, even if the rear calipers with the E-brake are SLIGHTLY out of adjustment, would that really affect pedal feel this much? I mean my pedal is a straight up limp noodle. And after re-addressing the e-brake adjustment multiple times, the "bite" has gotten better, but the pedal still feels like crap.


yes..there is no telling what the pressure was before...buy now its roughly a third less

unscrew the rear bleeders .....completely remove them.. make sure they are cleared and working and while there out use a vacuum bleeder...or pump the peddle and see if you're getting fluid thru the bleeder holes

[Tomswheels]

Seems like a vacuum or booster issue, could the new booster be bad?

[FLYNAVY53]

thats what Im leaning towards....going to put a vacuum gauge on it today and verify that Ive got enough vacuum at idle and go from there

[stevenp0827]

I just had the same situation hard when bleeding then would drop and go wet noodle when running on booster. I have a 69 camaro with the Cadillac conversion calipers. I had done everything exactly as you described , ended up using a longer rod between the booster/master. I think when it was running under vac. On the booster it was just placing the mc piston in the wrong place on its stroke . So maybe try a longer rod or even adj. your pedal at the column.? Good luck.

[Apogee]

In my experience, a bad booster or low vacuum would typically result in a high, hard pedal with a low brake-torque situation, not a low and mushy one. So while it certainly doesn't hurt to check the booster and vacuum levels, I don't think that's your problem. Low vacuum levels don't suddenly somehow cause the booster not to work, it just proportionally reduces the amount of gain or boost that it provides. While boosters will work very well at 18" Hg or more, they work reasonably well all the way down to 14" Hg in my experience and marginally well all the way down to 11-12" Hg. That said, booster size and configuration tends to be just as important, so an undersized booster (i.e. dual-7", single-9", etc) paired with low vacuum is certainly going to be less than ideal.

You want to make sure the booster output rod and MC piston has about .015" to .020" clearance. Too much and your pedal travel can be excessive. Too little and it may not allow the pistons in the MC to fully return to their home position, possibly maintaining residual pressure in the system.

Which CPP brake kits are you running specifically? Calipers and caliper piston areas? What bore size master cylinder? When you tested the MC by plugging both outlet ports, did it hydraulically lock after minimal pedal travel and not sink over time? When you reconnected the hard lines, did you do it one circuit at a time or both at the same time? That would at least tell you which end your issue is located, allowing you to focus your energies there.

Tobin
KORE3

[Skip Fix]

If they are the Caddy, 79-81 TA rear disc calipers that ratchet the ratchet is what keeps the piston and pads near the disc. They need to be ratcheted to where they grab the disc and then put on the bracket for the E brake. The E brake needs to be used almost every tome you stop to keep the pad close to the rotor as it wears. They are bad about locking up also and not ratcheting close enough.

[FLYNAVY53]

How did you determine the rod from the booster to the master was too short and how did you determine what length to use? Also, how did you remove the rod from the booster....Ill try to get a pic posted of mine here in the morning. Thanks for the help!

[FLYNAVY53]

I just had the same situation hard when bleeding then would drop and go wet noodle when running on booster. I have a 69 camaro with the Cadillac conversion calipers. I had done everything exactly as you described , ended up using a longer rod between the booster/master. I think when it was running under vac. On the booster it was just placing the mc piston in the wrong place on its stroke . So maybe try a longer rod or even adj. your pedal at the column.? Good luck.

How did you determine the rod from the booster to the master was too short and how did you determine what length to use? Also, how did you remove the rod from the booster....Ill try to get a pic posted of mine here in the morning. Thanks for the help!

[FLYNAVY53]

In my experience, a bad booster or low vacuum would typically result in a high, hard pedal with a low brake-torque situation, not a low and mushy one. So while it certainly doesn't hurt to check the booster and vacuum levels, I don't think that's your problem. Low vacuum levels don't suddenly somehow cause the booster not to work, it just proportionally reduces the amount of gain or boost that it provides. While boosters will work very well at 18" Hg or more, they work reasonably well all the way down to 14" Hg in my experience and marginally well all the way down to 11-12" Hg. That said, booster size and configuration tends to be just as important, so an undersized booster (i.e. dual-7", single-9", etc) paired with low vacuum is certainly going to be less than ideal.

You want to make sure the booster output rod and MC piston has about .015" to .020" clearance. Too much and your pedal travel can be excessive. Too little and it may not allow the pistons in the MC to fully return to their home position, possibly maintaining residual pressure in the system.

Which CPP brake kits are you running specifically? Calipers and caliper piston areas? What bore size master cylinder? When you tested the MC by plugging both outlet ports, did it hydraulically lock after minimal pedal travel and not sink over time? When you reconnected the hard lines, did you do it one circuit at a time or both at the same time? That would at least tell you which end your issue is located, allowing you to focus your energies there.

Tobin
KORE3

Tobin, the CPP kit Im using is 5760CBK5.5 front and CP2505-50-5512 rear. Unfortunately they don't list bore and piston size on the web site. They are the very base model, single piston calipers.

When I tested the master by plugging both ports, it did hydraulically lock with a slight amount of pedal travel with no sink over time. I did not check each circuit individually, which I suppose should be my next step tomorrow. Unfortunately I was unable to get out of my neighborhood today to pick up a vacuum gauge, or more brake fluid because the hotel across the street burnt to the ground this morning and they just opened the street up recently...what a waste of a free day off work......

I appreciate all the help!

[FLYNAVY53]

If they are the Caddy, 79-81 TA rear disc calipers that ratchet the ratchet is what keeps the piston and pads near the disc. They need to be ratcheted to where they grab the disc and then put on the bracket for the E brake. The E brake needs to be used almost every tome you stop to keep the pad close to the rotor as it wears. They are bad about locking up also and not ratcheting close enough.


Yup! After multiple hours, they're adjusted properly....drivers side is just under 1/8 inch of travel and the passenger side is just over 1/16 inch of travel. both sides have equal drag when you spin the wheels. Pedal still goes limp as soon as the truck is running.

[68byrd]

I ran into the same issue in my brothers c10. Does your truck have the parking brakes hooked up? His were not hooked up. So as a test I removed the big springs between the parking brake lever and caliper and then very slightly applied the parking brakes by pushing on the levers by hand then drove the truck. The pedal was great after that. Installed parking brake parts and got them adjusted. The brakes are working well now. It did 1,200mi of power tour this year and the pedal was spot on. The parking brake has to be used frequently to keep the adjustment tight.

[Apogee]

Tobin, the CPP kit Im using is 5760CBK5.5 front and CP2505-50-5512 rear. Unfortunately they don't list bore and piston size on the web site. They are the very base model, single piston calipers.

When I tested the master by plugging both ports, it did hydraulically lock with a slight amount of pedal travel with no sink over time. I did not check each circuit individually, which I suppose should be my next step tomorrow. Unfortunately I was unable to get out of my neighborhood today to pick up a vacuum gauge, or more brake fluid because the hotel across the street burnt to the ground this morning and they just opened the street up recently...what a waste of a free day off work......

I appreciate all the help!

Please measure the pin to pin distance on the front calipers. If it is 5.47" [139mm], then you have the GM D154 metric calipers. These calipers use a low-drag square o-ring seal designed to retract the piston in the caliper further than other conventional caliper designs, which means that they tended to work best with quick take-up master cylinders designed to account for the greater volume required to take up the slack in the system prior to actuation.

This design feature plus the inherently finicky rear Caddy calipers could very well be the bulk of your pedal feel problems.

Tobin
KORE3

[Skip Fix]

The Caddy/TA rear calipers are the same spread as a D154 if you need to do a cheap trial if it is those calipers. Oreiley or Autozone "plain" metric front calipers form a late 70s early 80s Malibu. Most have a 2.5" piston. I have them in place to the ratchets n the 79 Camaro race car

[FLYNAVY53]

Update.....

Decided to start over so I re-bench bled the master cylinder and reinstalled it. Just for giggles, I tested the pedal feel with the bleeding apparatus still installed and the pedal felt as expected. I then plugged both ports and tested the pedal....there was a very minute amount of slop in the linkage followed by about 3/4 - 1 inch of pedal movement before it locked up and would not depress further.

I then left the rear port plugged and attached the front to the proportioning valve. Pedal felt great with the truck off. With the truck running, and vacuum going to the booster, the pedal once again felt like a wet noodle. Turn the truck off, bleed off the pressure in the booster, and the pedal returns to feeling solid.

I then connected the rear port and plugged the front. Same story. Solid feel with no booster, felt like crap with the truck on and the vacuum line attached.

At idle, the truck pulls 19-20 in. of vacuum.

Any thoughts on what Im missing here?

[Skip Fix]

Sounds like a bad booster. Make the jump to full manual??

[FLYNAVY53]

Sounds like a bad booster. Make the jump to full manual??

Im with you 100% on going full manual....unfortunately, its not my truck, and its going to be daily driven by an 18 year old so I think a well sorted power system is the way to go. That being said, he insisted on keeping the 3 on the tree and manual steering with the 20" wheel....so good on him for keeping it old school.

I got on the phone with CPPs tech support, and given all the indications and trouble shooting, they're having me send the booster back to get tested/repaired/replaced....like you guys said, Im not convinced but at this point, its all Ive got left...I re-bled the entire system again, and re-verified the rear parking brake set up. The truck actually stops shockingly well given the pedal feel, or lack there of, but its certainly not right and I certainly am not comfortable allowing anyone to drive it on the highway. Still open to any additional suggestions.

[Skip Fix]

Just out of curiosity what size booster single or dual? Pedal ratio?

On the Trans Ams using that basic D52/D152 calipers used a 1 1/8 MC(quick take up in 81) and a 9" dual diaphragm booster and a pedal with LESS ratio than the normal 11" single booster disc/drum setups. When I first swapped it on my 78 I kept the same pedal as my disc drum. Little feel but stopped good until I bought a new GM correct 4WD pedal. So if you have a lot of ratio you might have poor feel if a dual booster.

Heck throw a manual MC on temporarily while the booster is out to check the feel! Or slide the master up(not tweaking the lines) and try it as a manual.

[gio75camaro]

Make sure your vacuum line is in your intake that also helps create more vacuum

[FLYNAVY53]

Booster is 9 inches. I guess another question to ask is do I need to change out the pedal? The original brakes system was unboosted drum-drum set up....

[Skip Fix]

You can just redrill the pedal hole higher for more ratio or lower for less and hook up the clevis there. Hole for boosted might already be on the pedal.

Single or dual diaphragm 9"?

[FLYNAVY53]

just measured....right now Im sitting at 6.25:1 which according to some google research is in the range for manual brakes but waaaaayyyy too much for power. Im 12.5 inches from the pivot point to the pedal, so I figure if I re-drill a hole at just under 3 and 1/4 inches I should be right at the 4:1 ratio for the dual diaphragm 9" booster

[Skip Fix]

My TA with the TA 4WD dual booster took less than the standard single booster ratio which is less than the 6:1 manual hole. But a good place to start. I'll try and stick my head up under the dash and measure this week for you, but since the temps are over 100 might have to wait until it is cool.

[a67]

just measured....right now Im sitting at 6.25:1 which according to some google research is in the range for manual brakes but waaaaayyyy too much for power. Im 12.5 inches from the pivot point to the pedal, so I figure if I re-drill a hole at just under 3 and 1/4 inches I should be right at the 4:1 ratio for the dual diaphragm 9" booster

That will do it. I take it this truck was originally a manual set up. Being a '60 and all...

At 12-1/2" pivot-to-pedal and a hole 3-1/4" from the pivot, that is a 3.8:1 leverage ratio.

For dead on at 4:1, make the clevis pickup point 3-1/8" from the pivot.

Note up top in the sticky (Sutton & Tobin?), I posted a brake calculator spreadsheet. Comes in handy for stuff like this.

Bob.

[FLYNAVY53]

yup....sure enough! Drilled a hole 3 1/8" from the pivot point, which necessitated moving the whole booster/master assembly an inch lower on the firewall, but once everything was bolted back up, I nearly put myself through the windshield from a 30 mph roll! Perfect pedal feel and a night and day difference in grip from the original drum set up. Thanks for all the help guys! Hopefully this thread will help others with similar issues.

[hotrodalex]

Where was the original hole drilled?

[FLYNAVY53]

2 inches from the pivot point

[Skip Fix]

Good deal! Back when 4WD were first coming out late 70s early 80s Hot Rod magazine article said they needed more ratio on pedal than disc/drums on the Trans Am setup but didn't mention the dual diaphragm booster added more assist so needed less. So I went through the same deal when I swapped them on my 78 TA. Gm still sold the 4WD pedal so that was an easy swap for me.