Back on forum need brake help

[SamIam]

Hello everybody, I finally got my 1968 CJ five with the V6 factory engine. I bought it from a good friend of mine and re-painted it overall it’s in great shape. It has a front disc brake conversion, all new brake lines, new front calipers, a new Wilwood check valve and a new proportioning valve. However, intermittently, I will lose the brake pedal one pump of the brakes. It restores the pressure anybody have the same problem?

[bmorgil]

SamIam, describe a little more detail. Are you saying you are driving down the road with everything OK with the brakes, and you press the pedal to stop and it goes to the floor? Then if you quickly give it a pump everything comes back?

I would look at a few things. First put it in the air all four wheels. With some one slowly applying the brakes, check the bias front to rear. The rear brakes should not activate until a good bit of pressure is being applied. The fronts should grab hard right away. If the above is what is happening, it sounds like you might not be getting the master cylinder piston refilled on the back stroke. If everything checks out OK with the check in the air, I am thinking the master cylinder has a problem. It could have picked up some crud blocking a port intermittently.

[VTWilly]

My 55 wagon does this if/when I let it sit for a bit. Not usually an issue if I am driving it every day, but if I stay home all weekend, when I get in Monday morning the first brake press of the day goes all the way to the floor. After that it works as you would expect. I just make sure to pump it a few times before I start.
I just sort of assumed that some of the fluid drained back into the master cylinder.

-VTWilly

[SamIam]

Thanks, I’ll try that. I replaced the master 2x making sure the
Reservoir for the disc brakes was larger than that of the drum brake rear. Still does the same thing. I can drive it 10 miles. It won’t do it go to pull into a parking spot. It’ll lose the pedal driving me crazy.

[gmwillys]

As Bmorgil stated, it sounds like you have picked up some crud that is hanging up the check ball that prevents the fluid from migrating away from the master cylinder piston. We say crud because it is intermittent from how the issue is described.

[TJones]

SamIAm does your brakes have a brake booster?
If it does I would check the vacuum line coming from the engine to the booster.
I’m guessing it has a Buick 225 “odd fire” in it, I would check to see if the vacuum line comes off the intake manifold or the carburetor. Sometimes those 225’s bc they are not an “even fire motor” will not make enough vacuum to operate a booster if the vacuum line comes off the 2 bbl carb and needs to come off the intake manifold.
I’m not sure what the vacuum needs to be to operate the booster correctly,but that may be part of your problem for the brakes to be inconsistent.

[bmorgil]

SamIam, if it did OK with the wheels in the air on the bias check, I would perform a thorough fluid flush/change. Using a pressure bleeder, if you can rent or use one, (or find a shop that has one) force several pints of fluid through the system with all the bleeders open. Something is preventing the return of fluid to the master cylinder. Either something is mechanically wrong with the master cylinder or valving, or there is something floating around in the fluid.