drum Brake problem need some help

first let me start bye saying thanks for taking the time to help solve my problem My 1938 Buick it is all the original style brake system i have installed all new brake lines rebuilt master cylinder rebuilt all wheel cylinders but when i drive the car for abought 30 min or so the brake pedal gets really hard the brake lights come on and stay on for abought 10 to 15 min then pressure goes away lights go off pedal is normal ,ive tryed adjusting the brake pedal rod rechecked all brake lines, i am wondering if the tail pipes are heating up the master cylinder cuasing the fluid to exspand i tryed a heat sheild but the dual pipes generate a lot of heat and the master gets kinda of hot could this be the problem may be i need to reroute the mufflers has any one had this problem before ?I AM AT MY WITTSEND thanks for any help

 

Is the brake pedal backed off to where you have 1 inch movement at the pedal before the brakes start to work?

There is a hole in the master cylinder that allows the fluid to flow back into the reservoir when you release the brakes. If the piston is held down it will cover the hole and cause the symptom you describe.

Or if there is gunk and trash blocking the hole.

It isn't so much brake fluid expanding, that is normal under use. The problem is the pressure is not releasing when you release the pedal.

 

Winner Winner checken dinner.

Look to the master for a problem, it probably has a piece of crap in it not letting it release the pressure from braking.

 

Did you also replace all 3 rubber lines in the brake system?

 

pasadena mine only has 1 rubber line conecting the the rear brakes to the main hard line should i have three?, i have a rubber line at each wheel, also im wondering if the piston from master is not coming back all the way like rusty mention, thanks guys it has to be from my master when i bleed them they all seem fine.ill do some more investaging,

 

yes rusty i have abought 1" play before braking also porknbeaner i have change the fluid twice and rebuilt the master twice again thanks guys

 

Do you have a return spring on the pedal?? The internal spring in the MC is NOT strong enough to return both the MC piston and pedal.

 

yes john i have a return spring , still locking hmmmm thanks for all your input i will eventually solve this problem

 

I do not think it is a heat related issue myself, the MC would have to be over 200* for it to be a issue and then it would be a spongy pedal like air in the system. But a simple heat shield over the pipe or on the MC should be pretty easy to fab.Pull the pushrod boot on the MC back and lay a eyeball on the piston, does it retract all the way to the snapring or whatever retains it when the pedal is operated?

 

thanks john will do

 

Each front wheel cylinder has a rubber line running from brak pipe fitting to the cylinder.
At the rear is one rubber line running from brake pipe on frame to rear end housing fitting which branches to each rear wheel cylinder.

 

i have seen on at least 5 occasions where the brake flex hoses act like a check valve-especially if rubber hose has a metal strap to retain hose to prevent rubbing.
if fluid is fresh,its clear and you should be able to see the ports on the bottom of the m/c.have someone move pedal slowly and watch to see if you can see parts moving.
i can see the pistons move back and forth in some master cylinders.
a piece of dirt may be restricting it.
when this happens,crack each bleeder to see where the pressure is building up.
it may narrow down whats happening.
ok,here's a thought how about cooling of master with icewater or a cold wet towel
would tell you if its heat or not.
as im thinking-i really dont want to mention it,as its impossible,but contaminated fluid will do this too-oil instead of brake fluid.but would get worse over time.
a quick-lube did this to a vehicle a few years ago,it was a little top-off but enough to create problems a few weeks later.destroyed every rubber component.

the reason this brake fluid is used,is its unique quality to resist expansion under extreme heat conditions.DOT 3 brake fluid is rated to 450 degrees,IIRC.

 

yeah all 3 have been replaced with new lines

 

Is it a floor pedal?? does the pedal have a return spring on it??? Had this problem before and the weight of the pedal would cause it to apply the brakes doing just what you described!!

 

yes i have a return spring , brake fluid is clean im sure it just seems odd that i can pump the brakes for any amount of time when car is sitting and no pressure build, but like i said you drive around for 30 to 40 mins pedal gets hard brakes begin to drag brake lights begins to stay on i let sit for 15 20 min im fine brake pedal will fill normal thats what makes me think that the mc is getting hot cuase it fills hot after a while,the dual exhuast runs abought 4" from mc but even running i dont think that the mc gets hotter then 450*degrees im thinking a bad mc even though this one seems fine im stump here fellas but thanks for all your input im going to relook at the mc and let u no what happens,