Plymouth P15 Brakes locked can't be driven

[Goodlet]

Problem:

The car cannot be driven because the brakes lock on after a few presses of the pedal.

 

I've put new rubbers in the drums and master cylinder.

 

I believe the problem is related to the return valve in the master cylinder but after adjustment per the manual the pressure release port is still inoperative although after much adjustment the port opens but not at the time it should.

 

Can anyone help me understand what's happening here and how to fix it?

 

Thanks!

[Plymouthy Adams]

the ports in the master must be clear and of most importance is the ability of the master cylinder to return to it full stop when the pedal is let up so that the piston will allow return of the fluid through the port.  Also check your individual brakes at each wheel for fully returning pistons and proper spring tension.  Dirty and sludge built up wheel cylinders can also mock a faulty master at times.

[Goodlet]

thanks will try this b ut before dis assembling are you able to advise which way the flow of the end valve is?  many thanks

[Plymouthy Adams]

do you have a manual?  this will show you the correct fitment of the internal pistons in the MC..the pedal push rod must be adjusted to have slight end play with the piston in the master fully against the stop when the pedal is fully return to non braking position.

 

the piston and cups in the MC bore are aligned with the lip of the cup facing the fluid outlet of the MC (not the pedal rod end)...this will allow for the ability to push fluid and as the pedal force and fluid resistance get greater it will cause the expansion of the cup so to get even a greater seal against the bore.  When there is no pedal pressure applied the brake fluid is no longer compressed, the cups relax the spreading of the lip and the internal master pistons are now free to travel back within the bore of the MC as fluid from the wheel cylinder flows back into the master and out the bleed hole and back into the reservoir.  If this event happens and your brakes shoes are adjusted for minimum .006 clearance, there is full pedal travel back to rest and the proper free play and there is no sludge in the individual wheel cylinders, your brakes will release. The return pressure for the fluid is from the springs that are on the brake shoes that keep them in constant contact with the shoes/cylinder via spreader push pin which is direct contact with the piston in the wheel cylinder.

 

again, a look to your repair manual would give you a clear understanding of the system.  If you do not have the book, I would recommend getting one as it is one of the most important items in your tool box especially when it involves working on the only true safety feature of the older car, brakes

[Plymouthy Adams]

oh..and another thing if you will, as there is no background information given by you,  did this just happen as you were driving the vehicle or is this a problem from a car that has been sitting a very long time..or even from a recent maintenance action as in rebuilding or adjusting the brakes. shoes can and do rust seize to a drum if car sits in a damp environment for a long period of time as can the very clutch disc also....people can over look proper sequence and adjustment procedures.  Again,,,the book is invaluable here.

[oldodge41]

Is it all the wheels? If so you are on the right track at the master cylinder.If it is only the rear or only one front brake it could be the rubber hose. I have seen them trap pressure like a check valve when they fail. 

[dale]

Replace all the brake cylinders, master cylinder lines and hoses.  Money well spent.

[grady hawkins]

While working the problem, you might also check the small pin hole on the shoulder of the master cylinder filler cap to be sure it is open

[Goodlet]

thanks to all my respondents. Sorry for the late reply 

 

background:

it happepend on the first test drive afterthe rebuild . the pedal got harder and harder until the car wouldnt move. The pedal has eased off since and the car was able to be driven up my driveway off the low loader .brakes are still 'on' though

 

further background:

the climate in joburg is desert ie dry hot and 6000ft high altitude. (perfect rust free climate) i have aworkshop manual.

 

Ithink what i want is to know if this is an adjustment problem or must i rebuild thewholr system (which has just been done)

Edited April 7, 2016 by Goodlet

[Goodlet]

further  p.s

 

the piston in the MC can be seen to move with the reservoir cap off and the fluid is spotlessly clean at the MC i think that the relief port is open but it is difficult to be sure. the pedal rod is coorectly adjusted.

[Don Coatney]

What you describe is a common problem when the pedal rod is not adjusted correctly. If the pedal rod is adjusted correctly when you release the pedal you should see fluid returning to the master cylinder through the small hole where I have a pin punch inserted in this photo.