need help diagnosing brake issue

[GallopinGhost]

Got a brake problem that has me stumped. I have a challenger stocker that I've built from the ground up. Everything in the brake system is new. The master cylinder and 4 wheel discs are from strange(I've replaced the MC once, no change). It has a hurst line loc, brake pressure switch, and a pressure gauge plumbed into the dash. No proportioning valve.
The problem is it seems to have air in the system. If the brakes sit for an hr or so, the pedal goes just about to the floor. Pump them up, feels great, an hr later, junk. I have pressure bleed them many times using quarts of fluid.

Here's the crazy part. To diagnose, I removed the brake lines from the MC and put in a loop. Pedal feels great. Removed the loop and plugged one port and re-connected one line. Pedal feels great. Put the plug in the other port and connected the other brake line, pedal feels great. Attached both lines, pedal is almost to the floor......

[SDT1DYI]

Are you using a 2psi check valve in the brake line? What size bore MS?

[pmrphil]

I've had issues in the past with the calipers not being "square" to the rotor - had to grind the spacers for the caliper brackets to get the caliper to be exactly parallel with the rotor when at rest, otherwise the caliper was pressure loaded at an angle and would push the pistons back in after a while.

[Dave1695]

First question, did you bench bleed the master before installing? If yes, did you push the piston ALL the way multiple times. If not, you have air in very nose of MC. Second question, is your line lock mounted below your MC ? If it's above MC air is trapped there and won't bleed out easily. Last, forget residual pressure valves if the MC is above the calipers, gravity works!

[GallopinGhost]

No check valves. 1.125 MC.
Yes on bench bleed. Everything is lower than MC

[340Cuda]

How have you bleed the brakes?

I have had best luck using a system that pressurizes the master cylinder and then liberally bleed fluid from each caliber.

I do use residual valves.

I have had problems in the past not too dissimilar to yours that I think are caused by the larger fluid volume of the rear brakes compared to the front.

Good luck and please share you fix when you find it.

[Mike Pearson]

First thing I would do is to make sure the calipers are perfectly square to the rotor. That is paramount to the brakes working correctly and not dragging. Then make sure the calipers are positioned correctly that the bleed screw is at the highest point of the caliper. After that I would take the pressure gauge and pressure switch out of the system and re bleed and check. Those two items would be suspect to not bleeding completely. You should not need a residual pressure valve.

[Al Wilson]

I had a similar problem on Strange rear disc brake conversion I did once. These were two piston calipers with a bleed valve on the top and bottom of the calipers. What I had to do was take the caliper off (with brake line still attached), turn it upside down and bleed the other end that was on the bottom. Problem solved.

[GallopinGhost]

On the caliper alignment, I have centered the caliper bracket to the rotor using shims, so they should be centered and parallel pretty good.


The pressure gauge that's on the front of the dash is mounted in a tee. the gauge is on the bottom and I have a bleeder screw at the top.


The calipers have bleeder screws top and bottom. I did try flipping them over and bleeding them, never saw a difference.


The issue has been going on since day 1, which was before the pressure switch and pressure gauge. I have flipped the lines between the front and back ports on the MC to get better holding power on the starting lines, didn't see much difference. Once pumped up, I can get 1000 psi on the gauge.

[GallopinGhost]

I use a pressure bleeder which really is a nice piece.


Right now the rear brakes are connected to the rear port on the MC....

Originally Posted by 340Cuda

How have you bleed the brakes?

I have had best luck using a system that pressurizes the master cylinder and then liberally bleed fluid from each caliber.

I do use residual valves.

I have had problems in the past not too dissimilar to yours that I think are caused by the larger fluid volume of the rear brakes compared to the front.

Good luck and please share you fix when you find it.