Disc Brakes, Pressure build up?

[rocmoc]

I installed a complete '78 CJ7 Dana 30 diff with Disc Brakes in my '69 CJ5 . Everything has gone perfect but one little problem. Pressure builds in the disc brakes after a little use. Bleed off the pressure (left front, but no pulling) and everything is fine. I installed a new '69 master but made sure the drum pressure valves were removed. I used the '78 proportioning valve & brake lines. Calipers were in good shape & new pads were installed. Any ideas? Bad proportioning valve?

Thanks,
rocmoc n AZ/Mexico

 

[Purple77CJ7]

I would check your calipers. You mention the left front, it sounds like the piston is sticking in it's bore. Could be junk built up behind piston. I would remove caliper and try using a c- clamp and run it all the way in. If it has sat for awhile could also be a ridge on piston that won't let it move free. Might be a bad piston o-ring sticking,too.

 

[gert]

Also check your hose, when they go bad they will hold residual pressure as well, it is weird the inside of the hose constricts the flow

 

[rocmoc]

Thanks guys. New hoses but I am going to check for kink as they are longer extended hoses. Was also thinking about the fact that it is the closes caliper to the master that requires bleeding. Normally it is the furthers so next time it happens I will jack up the front and spin the tires to verify if it is one or both.

thanks again, anymore ideas?

rocmoc n AZ/Mexico

 

[jasonmark1993]

Does the rod from the pedal clip into the master?

When I did my hydro clutch, I ended up with a similar situation with a very small amount of pressure at all times. Turned out that the rod that came with the new clutch master was a little big and when it was c clipped into the master, it was depressing the plunger a little.

But the stuck caliper is a good spot to look too.

 

[tarry99]

I installed a complete '78 CJ7 Dana 30 diff with Disc Brakes in my '69 CJ5 . Everything has gone perfect but one little problem. Pressure builds in the disc brakes after a little use. Bleed off the pressure (left front, but no pulling) and everything is fine. I installed a new '69 master but made sure the drum pressure valves were removed. I used the '78 proportioning valve & brake lines. Calipers were in good shape & new pads were installed. Any ideas? Bad proportioning valve?

Thanks,
rocmoc n AZ/Mexico

1969 CJ...........do you still have the floor Pedals and the original single chambered frame mounted Master Cylinder?
If so, that's your problem.
For a Disc / Drum setup you need a dual chambered Master cylinder.
Also because the MC is below the level of the Calipers & wheel cylinders a 2 lb. Residual valve for the Disc's and a 10 lb. Residual for the drums is needed & can be had with the correct MC.

Also its doubtful that the floor pedal ratio is great enough to pump adequate pressure for the Disc's.............Knowing the MC bore diameter & stroke along with total volume of your system is important to Spec all the right parts.

 

[jdcaples]

A little more on hoses:

The flexible portions of brake fluid lines are - over time - susceptible to internal delamination, or peeling apart on the inside.

Flaps of brake hose peeling off the inside walls of the hose can cause a check-valve effect, where fluid travels toward the caliper or wheel cylinder easily but encounters resistance going the other direction.

In the end, the shoes or the calipers remain pressurized, potentially glazing the braking surface(s).

In my dad's brake shop ('72 to '88) we saw this type of failure far more often out-n-out hose ruptures.

-Jon

 

[rocmoc]

New extended hoses & new double chamber master. I'm going to check the caliper this weekend. The calipers were the only old part that was reused.

Thanks everyone!

rocmoc n AZ/Mexico