Manual Master -- not enough volume?

[Ethereal]

I converted to a manual master and am experiencing an inability to lock up my brakes. The car (68 chevelle) does stop adequately but I feel that I'm leaving some stopping distance on the floor. I'm running disc/drum with a prop. valve and I had to dial the rear down quite a bit to ensure that it doesn't lock up.



Specs:
- Stock 11" discs front, stock drums rear
- Stock D52 calipers with thermoquiet pads front, ancient shoes rear
- 7/8" bore master (mopar)
- 1" of pedal stroke, 1:6 ratio
- 10lb residual valve to rear drums
- 3/16" line all around and stainless braided hose

I know I could get some pads with a higher friction coefficient but clearly I'm not looking to race this car (yet) so I went for longevity in the pads. I bled my lines a couple of times and bench bled the master until it was rock hard, so I'm confident there's no air in my system.

Am I just running out of volume when bottoming out the pedal? Unless I'm mistaken it should take extremely little volume to engage the front pads. I added a residual valve to the rear thinking that the return spring was pulling the shoes too far off the drums, requiring additional volume to engage them but that had no perceivable effect. My next step would be to move to a 15/16" bore MC but before I do that I wanted to make sure I'm not overlooking something. Anything I could check that I didn't already mention?

Or, am I just overly optimistic about my braking system? My calculations give me about 1700lb of total braking force assuming .3cf on those thermoquiets (internets say GG rated but mine have no markings...).

[timopajala]

How well did you air the system? I had previously great problems bleeding the rear brakes and had week braking power, then i used vacuum pump and m friend game to help me with it, after that the brakes work very well. The conjunction peace held air in it and normal brake pedal pumping did not make it move anywhere.

[Ethereal]

You mean the distribution block had air in it? I was thinking about it last night and I'm pretty sure the front and rear circuits are completely independent, so I really just need to focus on the front circuit. I did use a mityvac on the rear but not the front, I guess it couldn't hurt to try that but I'm getting zero air bubbles out of the front with the pedal-pump method.

Maybe I'll have someone pump the pedal and then crack the fittings at the distribution block to see if I can get some air out at that point. I'm not really sure how pressurizing the system purges air though; wouldn't the air want to stay at the highest point in the system regardless of pressure?

[andrewb70]

I have found that one of the simplest ways to bleed brakes is use gravity. Crack every bleeder just a little and keep topping off the MC. I run a 7/8" MC in my GTO with C6Z06 brakes in the front and LS1 rear brakes. Not sure how that relates to you, but it's a datum point.

Andrew

[Dark Pursuit]

Make sure that when bleeding rear brakes that you open an adjustable proportioning valve fully during bleed. If you count turns to full open before adjustment, returning to your adjusted proportion is relatively easy.

[Ethereal]

I did actually gravity bleed first, that's always my first step after bench bleeding the MC because it's so easy I did have the prop. valve all the way open when bleeding the rear the first time, BUT I think I've ruled out the rear brakes being of any importance to this issue (unless someone knows otherwise) because the problem is that the front brakes are so much weaker than the rears! I had to dial down the rear pressure a lot to ensure that I wasn't locking the rears on panic stops.

I hope that there *is* air in the line somewhere because I don't want to have to install a new MC, but the only place it could be as far as I can tell is the front portion of the distribution block. I've convinced myself that the volume of fluid to the front circuit is ample, so my plan now is:

- Make sure the MC is level, and plug the front outlet to re-bench bleed the MC while on the car -- just in case
- Re-bleed the front calipers because I just put air into the lines when bench bleeding
- Bleed the fittings on the distro block, including the warning light I guess because why not


Questions:
- Should I bleed the calipers first or the distro block?
- Shouldn't the amount of air in the distro block, if any, be really small and therefore inconsequential compared to the volume of fluid pumped into the circuit?
- Is there anything like a plastigauge of sorts that I can put in between my pads and the rotor to measure the actual force applied by the caliper?

[IMPALA MAN]

I used to run into this all of the time. Let me "brake" it down for you..lol. Ok when you depress the brake pedal, it pushes X amount of fluid into the calipers, which in turn pushes the caliper pistons outward, clamping the rotor. Short and simple, when you depress the piston in the master cylinder all the way, it must push out more fluid than it takes to fill up both caliper piston bores. If not, you will have little or no brakes.
Example: Most master cylinders have a 1" stroke. Try a 1" master cylinder and see what happens. Of course this is after you have done the math to check and or ruled out that there is no air in the system. Teh volume of your master cylinder calculates to .79 square inches. If it takes more than that to fill up both of your calipers, it will not work. Hope this helps.

[dontlifttoshift]

I converted to a manual master and am experiencing an inability to lock up my brakes. The car (68 chevelle) does stop adequately but I feel that I'm leaving some stopping distance on the floor. I'm running disc/drum with a prop. valve and I had to dial the rear down quite a bit to ensure that it doesn't lock up.

Specs:
- Stock 11" discs front, stock drums rear
- Stock D52 calipers with thermoquiet pads front, ancient shoes rear
- 7/8" bore master (mopar)
- 1" of pedal stroke, 1:6 ratio
- 10lb residual valve to rear drums
- 3/16" line all around and stainless braided hose

I know I could get some pads with a higher friction coefficient but clearly I'm not looking to race this car (yet) so I went for longevity in the pads. I bled my lines a couple of times and bench bled the master until it was rock hard, so I'm confident there's no air in my system.

If the problem is that the car doesn't stop to your liking, then the pads are your problem. 6:1 with a 7/8" master is generating plenty of pressure.

Am I just running out of volume when bottoming out the pedal? Unless I'm mistaken it should take extremely little volume to engage the front pads. I added a residual valve to the rear thinking that the return spring was pulling the shoes too far off the drums, requiring additional volume to engage them but that had no perceivable effect. My next step would be to move to a 15/16" bore MC but before I do that I wanted to make sure I'm not overlooking something. Anything I could check that I didn't already mention?

Or, am I just overly optimistic about my braking system? My calculations give me about 1700lb of total braking force assuming .3cf on those thermoquiets (internets say GG rated but mine have no markings...).

Bottoming the pedal on what? Is the pedal going to the floor? If yes, you will have to upsize the MC. Stock manual brakes would have been 1" bore if I remember right.

Side note. You were correct to install a residual valve, all drum brakes require a residual valve either inline or built into the master cylinder. The residual valve has nothing to do with the springs and everything to do with keeping the cup seals in the wheel cylinders properly loaded against the bore. Without the valve the seals will relax and either draw air or leak....if you're lucky it will do both. ;)

[Ethereal]

I used to run into this all of the time. Let me "brake" it down for you..lol. ... Teh volume of your master cylinder calculates to .79 square inches. If it takes more than that to fill up both of your calipers, it will not work. Hope this helps.

Bonus points for bad puns I haven't done the math tbh but let me see what we get...

Volume of fluid moved out of a 7/8" cylinder with 1" stroke:
1 x 3.14 x .4375 x .4375 = .6 cubic inches

Area of *both* caliper pistons, with diameter of 2.95":
2 x (3.14 x 1.475 x 1.475) = 13.66 square inches

Caliper piston travel:
.6 cubic inches / 13.66 square inches = .044 inches

Wow! Maybe I really am running out of volume! Let's see how a 1" bore MC would stack up.

Volume moved out of a 1" bore MC with 1" stroke = .785
Caliper piston travel = .0575 inches

Ok well, that's still a really small distance so now it's not clear whether volume is actually the problem. It is a 30% increase in travel though...

Bottoming the pedal on what? Is the pedal going to the floor? If yes, you will have to upsize the MC. Stock manual brakes would have been 1" bore if I remember right.

Yes the pedal goes to the floor. Any idea how far a stock manual pedal would go before becoming super firm? I'm still wondering if I do have air in the line vs. just running short on fluid volume, but it doesn't look like I'll be able to re-bleed tonight.

[Dark Pursuit]

If volume is a problem, I resolved this issue on my jeep years ago. (Big tires and rock protecting metal make for big braking requirements)
1978 Mercury Grand Marquis with 4 wheel disc brake master cylinder pushes massive volume and will bolt right up. Its overall size is about 1 1/3 size of most masters. cost remaned is about $30.

[andrewb70]

If volume is a problem, I resolved this issue on my jeep years ago. (Big tires and rock protecting metal make for big braking requirements)
1978 Mercury Grand Marquis with 4 wheel disc brake master cylinder pushes massive volume and will bolt right up. Its overall size is about 1 1/3 size of most masters. cost remaned is about $30.

Overall size has nothing to do with piston diameter or stroke...

Andrew

[Dark Pursuit]

I only mentioned the size in case it posed a space issue.

[a67]

Any idea how far a stock manual pedal would go before becoming super firm?

Just a data point, the GM spec for a manual '80 f-body is with 50 lbs of pressure on the pedal the maximum movement is 3.5" This is the at pedal movement from full release to the location with 50 # of force on it.

Bob.

[Ethereal]

Just a data point, the GM spec for a manual '80 f-body is with 50 lbs of pressure on the pedal the maximum movement is 3.5" This is the at pedal movement from full release to the location with 50 # of force on it.

Bob.

I have no way to apply 50lbs of force to my pedal (or do I... fishing scale might work!) but that's a good data point. How did you know that might I ask?

[andrewb70]

Put a bathroom scale between your foot and the pedal.

Andrew

[Ethereal]

Put a bathroom scale between your foot and the pedal.

Andrew

I only have a digital but that's a great idea! Maybe I'll try to grab one at the thrift today.

[Peter Mc Mahon]

Is the pedal rigid enough? It is not flexing the firewall excessively?

[Ethereal]

The firewall is not flexing. I did re-bleed the brakes at all fittings with no change in performance.

Since I can bottom out the pedal and with not a ton of effort, I think at this point I need to step up from my 7/8" MC to a 15/16" or 24mm MC and observe the difference. I might only have a 1" on hand though, as iirc the 24mm I purchased actually came as a 1" when measured. I'm tired of bleeding brakes though so I think I'll just drive it for a while as-is and change it out later!

[malibudave]

The D52 calipers have the largest piston area of most cars at about 6.83 square inches per caliper
To put how much larger the piston are is, let compare it to other calipers.

6.8 sqin - D52 caliper. 2.95" diameter piston
4.9 sqin - D154 metric caliper. 2.5" diameter piston
4.9 sqin - LS1 Camaro twin piston front caliper. 45mm diameter piston
4.0 sqin - C5 Corvette twin piston front caliper. 40.5mm diameter piston

I know that the D154 metric caliper was designed to run a 7/8" bore master cylinder.

The D52 caliper is roughly 40% larger than the D154 metric caliper. The D52 calipers where used on manual brake Chevelle and Malibus in the 70s. These came with a 15/16" bore master cylinder. So a 15/16" bore or a 24mm bore master cylinder should work well in your application.

A good street pad that works well cold will give better street manners than a pad that needs some heat before it starts to work.

[Ethereal]

Thanks Dave, that's good info! I actually have a set of the D154 calipers on hand for my truck so I'll give the 7/8" master a go with those when the time comes.