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my fronts brakes lock up after a short drive
i just purchased the truck its a 81 4x4 and i am working out the bugs.
so my problem is in the title and when they lock up if i let the truck sit for a couple hours the brakes unlock i need to find out if it the master, the booster or the preportianing valve. i was wondering if any one can help me out its my only vehicle now that my bugs engine took a crap, i need to fiqure this oout before moday cause i have no other vehicle.
thank you very much
Mike

Try bleeding the brakes so you have fresh clean fluid in the system (gets the moisture out).

Also check the front caliper mounting and slide to make sure they are not rusted so the calipers move freely

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1978 pro street step/side 455Buick, 4sp,4.11s, 31/18.5s

the front calipers are brand new and i had the rotars resurfaced i drove the truck around and let them lock up i releaved the pressure off the fronts and the unlocked

Sometime old brake hoses start to work like one way valves.

i am going to get a new master cylinder when i pulled the old one i had take off the resivoure and when i did the was alot of dirt in side it

100%

replace brake hoses

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zyanug
75 luv stepside... Work in progress
76 fleetside 350hp 1968 327cu
"No replacement for displacement!"
___/__l____
[_@l__l__@_]***

yepper...

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If you think no one cares, try missing a couple of payments...



FIAA!

SFPP!

well i replaced the master and it still drags the front brakes. i called les schwabs and the tech there said it could be sticking calipers (bear in mind mine are brand new) or a contaminated system. he said i would have to replace everything and clean all the lines. but i doubt that it is that because i have well bled the brakes and all the fluid comeing out is clean fluid. i quess my next step is the replace the flex hoses like you guys say and see if that works. if any one has incounterd the same thing i have id like to here from you

My wifes 'yota did the same thing a few years back. I replaced both front brake hoses and never had another problem.

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1962 Chevy C-10 Stepside-Rat Rod
1990 Chevy Suburban

The only way to combat violence is with greater violence!
Trying to please everyone is way to hard, but pissing everyone off is a piece of cake!

That would be my guess as well, seen it many times.

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Certified pilots, looking down on people since 1903.

my dad motorcycle did it too bad lines i think if i rember right

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77-luv-stock (Selling or Parting out motor blew up)

75-luv mikado-V8 350 soon

if i pumped the brakes with out running the truck or driving it. i should come up with the same problem right.

I just recently had the same problem. Just bought my truck. It was sitting for over 10years. It would run fine cold but as soon as it warmed up with out even touching the brakes it would struggle and slow down to @ 20-30mph. Going down hill it would not even coast. I bleed fluid out of the master and calipers, didn't work. It got worse to the point it would not even move when it locked up. If I let it cool down it would unlock. New brake pads had been installed on the old calipers. I replaced the calipers and reused the brake pads. Had a lot of trouble find them locally. Everywhere I checked I had to order them and wait min 3 days. Got them same day from a friend who check his parts supplier.

Truck has not had the problem since.

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Hello,
I had the very same exact problem with 1 of my luv trucks. The front brakes would lock up to the point where the truck couldn't move. I'd pull over and crack the caliper bleeder screw. The pressure would bleed off and then the truck could move again until I used the brakes again. I finally figured out that the brake booster had gone bad. It would slowly apply pressure to the master cylinder which in turn would seize the front brakes. To verify it was the booster I removed the vacuum hose and plugged it. Drove the truck again for 2 days without power assist and the front brakes didn't seize anymore. I then replaced the booster with another from 1 of my other trucks and didn't have anymore problems. Just my 2 cents.
Mike.

I guess my vote is for the booster this time. Yes the brake hoses go bad like that and it is a much more common repair.
However the chance of both front hoses plugging the same time is remote. I have usually had just one or the other screw up.

i doubt it the booster because i pumped the brakes with out turning the truck on and the problem persisted and i checked the booster by when i press my foot on the brake and turn the truck on it depressess a little and it doesnt hiss at me or squeek. one thing i was reading was that the pin that comes out of the booster that plunges the master is adjusted wrong, i was wondering that when i install the master to the booster should i have to force the master to the booster to bolt it up or should it be easy. because when i installed the master i had to press the master in some so i could get the nuts threads started. if thats not right id like to know how much play i should have before the pin touchs the master. i would like to talk to the person that had it before me so i could slap him and asked what was he thinking, but we all wish that some times

Master Cylinder or booster is my guess since you have replaced the wheel cylinders and bled the system and are getting clean fluid through it. That's my pennies worth.

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In life there is the "Way things should be" and then "There's the way they are".

2005 Silverado 3/4T Crew Cab
1981 Luv 2WD
1979 Luv 4x4 Currently going through Open Heart and Cosmetic surgery
1995 Roughneck JetBoat
1981 Luv parts trk

Not sure but I would think the pin should not push the master cylinder in or it may be applying the brake a bit.
The adjustment goes like this: With the brake pedal all the way up the distance should be 6" from the brake pedal pad to the floor pan (floor). You adjust this with the brake switch position. At this point the brake pedal push rod fork should have a bit of play, in other words the push rod to pedal connecting pin should slip out easily with no pressure against it. If not loosen the locking nut and turn the rod in or out as necessary then slide the pin back in. In other words with the pedal at rest it should not be pushing in on the booster/master cylinder. You may have stumbled onto something here that will fix this.

I agree with egg. When the master cylinder returns to rest, it exposes a hole inside it that releases the pressure to the calipers. If the master cylinder doesn't return all the way back, the pressure is never released. Proper adjustment of the push rod to the master cylinder should fix it.

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You went there in that?!?!

i found out that the pin from the booster to the master must have been adjusted to far out because when i took off the master and adjusted that pin it made a world of difference now i just have to fine tune it to get it right. the guy i bought it from said that the guy he bought from that his son worked on the brakes. so i assumed it was never messed with but then i rememberd the assume makes an ASS out of U and ME.