Looking for some help with my recent brake problem

[benno505]

in my 68 charger i have Wilwood:
* dual tandem master cylinder
* 4 port prop valve
* 12" drilled and slotted discs
* 4 piston calipers
* all brand new stainless brake lines and stainless flex lines.



Car has been driving perfectly for a few weeks, then yesterday driving down the highway and the brakes locked on, the brake pedal was hard as a rock and would not budge
managed to unwedge them and get the car home.

pulled master cylinder and brake booster off tested both and they both working fine, put booster back on and checked pedal movement, all good
put master cylinder in vice and ran the return hoses and bench bled the MC, it worked fine.

put MC back onto booster and reconnected the brake lines up and brake pedal is still hard as a rock.

Any ideas?

Thanks

[Schwartz Performance]

Did you bleed the brakes too? Maybe something is seized up front or back... ?

[benno505]

Did you bleed the brakes too? Maybe something is seized up front or back... ?

yeh brakes were all bled, i have done about 500miles on them they were working perfect.
then all of a sudden they locked up, on the side of the road we undud the booster and wiggled everything around which freed up the brakes, which allowed me to drive it home but the pedal was hard as a rock and brake pressure was very minimal.

do you think maybe a prop valve problem?

[Schwartz Performance]

Could be. I was asking if you bled the brakes after you reinstalled everything and checked the booster and master.

[MonzaRacer]

Sounds like either a booster issue, or incompatibility of master cyl to booster OR if booster isnt for that body style then you may have some sort of angularity issue locking it up.
Had several people match up incompatible parts before.
there really should not be any preload on the master cylinder on most when bolted to booster thats hooked to brake pedal.
This is where booster pushrod needs to be adjustable. Bolted to car push rod should slide on to pedal pin without preloading the booster. if master cylinder doesnt sit nearly flush after installation preloading or loading of the master cylinder piston as you bolt it down can cause issues. Most boosters either have adjustable pushrods or various lengths. You can also make a spacer shim.
Had one guy with booster that pushed master about 3/16 of inch,,,,several applications later it wasnt releasing calipers. Biggest thing is figuring out where the release point is in master cylinder when released.
Might try a spacer(washers) between master and booster to see if this is what is happening.