brakes locking on 78 c20

[bns454]

Hi i have an issue with my brakes locking on sometimes.
when it happened today i unbolted the master cylinder from the booster to check if the cylinder was jammed it wasent.
i then cracked the front and rear lines at the cylinder both sprayed fluid out and the brakes released
the brakes then worked as normal.
I am thinking i may have a problem with the valve that all lines go into mounted to the frame below the radiator? What is the purpose of this valve? Should i open it up and have a look? Or could it be somthing else causing my problem

[cadillac_al]

What's locking up a rear brake? It's usually a brake fluid or gear oil leak on the shoes.

[bns454]

All brakes are locked on front and rear
the first time it happened i geared down and drove the 2 miles home in first all four corners were smoking as soon as the lines are cracked its fine for a few days then happens again

[homemade87]

Ok thinking out loud here , The master cylinder is the common between all four corners . The proportioning valve fluids are separate from front to rear . The fronts use the same fluid ( left , right ) . The rears use there separate fluid from the front (prop valve ) . The prop valve only lets you know if the front or rear are no getting the pressure over the other . My guess the master or booster is hanging up and leaving pressure at all four corners . When you let the pressure off it releases the hang up .

Next time it happens pull the check valve out of the booster and see if it releases . If it helps it will confirm the master or booster or possibly a peddle problem .

[bns454]

That makes sense i have unbolted the master cylinder from the booster and they have stayed locked on
when i looked into the rear of cylinder the spool was against the circlip but maybe the spool is in 2 peices and im onley seeing the booster rod locating cup
Ill strip it down tomorrow and see
I was unsure if the valve was a prop valve or somthing else

[f.monroe]

If you unbolted the master cylinder and they were still locked then it's not the booster. If it unlocked when you cracked the lines then the master cylinder is locking up. I'd start there.
Frank

[Shugalou]

Based on what everyone had already stated I would venture a guess that the plunger isn't returning when the pedal/push rod comes off and it stays pressed in forcing the fluid to keep the calipers and wheel cylinders in stop mode.

[enaberif]

Originally Posted by Shugalou

Based on what everyone had already stated I would venture a guess that the plunger isn't returning when the pedal/push rod comes off and it stays pressed in forcing the fluid to keep the calipers and wheel cylinders in stop mode.

Yup. Time to pull the master cylinder and clean it out and see if it needs replacing.

Its just one small c-clip to hold all the parts in for the master cylinder.

[Desert1957]

Check the rear brake hose, I have seen front and rear hoses collapse internally and not release pressure.
If you crack the rear bleeder and the brakes release you found your problem.

Desert