Added rear discs, now no pedal

[ho428]

Added single piston rear Calipers, standard drum-disc conversion kit.
I have 1/4" steel lines from the MC, through a prop valve and SS flex lines at the axle.
No matter how much I bleed I cannot get a firm pedal without pumping the pedal up. Bleeders are mounted up, no air in the lines.
Does this not just on the track but sitting in the garage, pump-pump-pump, good pedal, let sit for 10-15 seconds and nothing, like the calipers are pushing the fluid back into the MC. Pedal stays firm as long as your foots on it, but take it off for 10-15 seconds and no pedal.
Seems a volumn issue, not an air issue. 1" bore disc-disc Corvette MC.

Going to add a residual valve and maybe springs behind the pistons, but was also wondering if I should change to a 3/16 main line to the rear. Read somwhere bigger isn't better for brake lines.

Any other ideas?

[Nessumsar]

Did you bench bleed the master?

[jknight16]

The 1/4" vs 3/16" line won't make a big enough difference to create the issue you're experiencing. I also don't think residual pressure or springs will solve your issue as you're not experiencing "knockback" per se.

[DynoDon]

Line diameter is NOT the cause. I am betting that there is air trapped somewhere in the system, even though you may not be seeing bubbles at the bleeder..

Unless you have a below floor mounted master, i'm also betting that residual pressure valves won't cure your problem.

[ho428]

I ran this same set up (MC-Prop-line size) for over a year with rear drums, no issues, firm pedal.
All I changed was the rears from drum to discs. Now no pedal.
Like I said it acts like when you release them it's pushing fluid back so you have to pump extra fluid in the calipers, similar to knockback.
It's got me baffled.

[MrQuick]

I ran this same set up (MC-Prop-line size) for over a year with rear drums, no issues, firm pedal.
All I changed was the rears from drum to discs. Now no pedal.
Like I said it acts like when you release them it's pushing fluid back so you have to pump extra fluid in the calipers, similar to knockback.
It's got me baffled.
should not happen with system full of fluid and no air.

Now did the pedal go to the floor at all during the breeding process? It is possible to damage the seal when this happens.


I usually replace the 1/4" line for 3/16" when swapping to 4 wheel disc but you should get a pedal. Im gonna echo here. air in system, air in master or faulty master.

[MonzaRacer]

Add in 2 PSI residual pressure check valve in the rear line. The system may require one to keep the pads out far enough. to prove the point remove the calipers and lock them all the way in with c clamps. If it goes away its elsewhere. if this cures the issue its the system releasing too much and your having to push the pads back out to get a pedal.

[Skip Fix]

Depending on the size of the caliper pistons and the size of MC might not be moving enough fluid without multiple pumps.

[Apogee]

Gordon, post up what the application is so that we know what you're working with. Include year/make/model so that we know what front brakes you're running (assuming OE) as that could be helpful. If you're not running OE style front brakes, what are you running? Are you running manual brakes or power?

What rear calipers are you using? You can get similar symptoms from either the rear Cadillac style parking brake calipers if the parking brake isn't properly adjusted during installation or the low-drag GM small metric calipers (fronts) due to the distance that the square cut seals retract the pistons in their bore.

Line size has no effect on getting a pedal, only on the firmness of the pedal due to the additional compliance of the system due to larger tubing diameters.

Tobin
KORE3

[ho428]

1968 Firebird, 72 Corvette MC. Road Race.

Running 1 7/8 bore 4 piston OEM Fixed front calipers.
Rears were the Caddy style but I sent them back for the non E-brake type for the problem you discribe.
I'm not sure what they substituted it with but it's similar in size to the Caddy, uses the same hardware to mount.

I ran the MC for a year with no issues, just had to manually adjust the drums a lot. I do have a new Wilwood 1" tandem MC to install to drop some weight. Thought I'd put it in when I install the 2lb residual.

Several guys at the track have said they added the valve and springs behind the caliper pistons, my fronts have springs, don't think the rears do.

Rears
https://static1.pt-content.com/images/pt/2010/11/100_3844-1.jpg

[MCB Matt]

What are you running for a prop/combo valve? We typically never run a resid valve with rear disc they should stay adjusted without.

[ho428]

Manual adjustable prop valve from Summit racing, got it mounted by my seat for adjustments during the race.

[MCB Matt]

Are you coming right off the master and running a "T" for the fronts?

[MuscleRodz]

Manual adjustable prop valve from Summit racing, got it mounted by my seat for adjustments during the race.
is it mounted on the floor or on the tunnel? you do not have a volume issue with those single piston rears. what you may have is a bad master

[Skip Fix]

The "metric" GMs bolt on in place of a Caddy/TA rear ratchets. They have a moderately big piston. They are generally the "low drag" and the factory used a "quick take up" MC for the 81 TAs using low drag calipers. MC size was also 1 1/8". Not sure how the 4 piston caliper in front compare to a 2 15/16" front caliper on the TAs for volume. Drum cylinders need alot less fluid to move them.

FWIW Wilwood said they were going to come out with a "metric" similar to their dual piston D52 the end of this year.

[Fuelie Nova]

Before I went with a Baer brake setup I had a homemade 4 wheel disc with the caddy calipers in the rear. Had the same problem. I unbolted all four calipers from the brackets and let them rest on the rotor making sure they were free to move.
I had a solid pedal when the calipers were not bolted to the brackets. Turned out my brackets for the rear were not square with the rotors causing flex when using the brakes. Machined the brackets and back to a solid pedal.
You can also clamp off the rear flex hose as a test to see if the problems are with the new rear setup. When I initially could not get a firm pedal I put a vise grip on all 4 flex hoses at the same time releasing one at a time. With the vice grips off the front but still installed on the rear I had rock solid pedal. This proved it was not the m/c etc. I know people will say not to put a grips on the hoses but when your desperate its an easy test. I was more than willing to replace the flex hoses if needed to prove the problem.
Keep us posted
Tom

[Hydratech®]

Fuelie Nova brings up an excellent point about the caliper brackets not being square to the rotors. You can loosen up the caliper support bracket mounting bolts and / or the caliper mounting bolts to see if this helps in cases like this. Before doing this, have a helper pump the pedal some while you watch for unusual caliper movement, like straightening up square in relation to the rotor during brake apply, then cocking back to a slight angle during brake release.

Here is our brake bleeding article that may help: http://hydratechbraking.com/braketech1.html

Let me ask the question that I haven't seen here in this post and can't tell in your pic: Are the bleeder screws at the tops of the calipers? (mentioned in the linked article) Also, you should snug the rotors down to the axle shafts with lug nuts if bleeding with the wheels off. Axle shafts that aren't dead straight (slightly tweaked) can also cause this situation. With the rotors secured down to the axle shafts with lug nuts, fire the engine and pop it in gear to see if the axle shafts and rotors are spinning dead true and straight -if not, this may progress into obtaining and installing a fresh set of axle shafts.