47-55.1 47 thriftmaster stock brakes issues

[jeffbcn]

Brakes are locking up the pedal starts out about three or four in from the floor but as I drive it it gradually Rises up until it's at the very top hard as a rock and the brakes drag anybody know what the problem is?

[GreasyLikeaBurger]

check the adjusting rod for the master cylinder. It may be to tight causing the master to be slightly pressed. And as the brakes heat up, the fluid might expand a little, causing the pedal to rise up.

[mr48chev]

A BIG problem on brakes on these trucks is a missing return spring on the pedal that causes the exact same issue as the too tight adjusted push rod that GreasyLikeaburger mentioned. Either way the piston is pushed in so the return port to the reservoir is covered (sorry no picture tonight my laptop hasn't recovered from the shock of a power outage and surge yesterday. The covered port doesn't let the fluid return and every time you hit the brakes it acts the same as pumping up the brakes. Problem is that after a while the brakes don't release because there is too much fluid trapped in the system.

[jeffbcn]

The brake return spring was intact and functioning the rod on the master cylinder has no free play I put a little free play in it and now the pedals kind of low so I know I have to play with how much free play goes in there to get my pedal right

[Wrenchbender Ret]

You don't use the master cyl. rod adjustment to "get the pedal right". A small amount of freeplay is correct. If the pedal is low the brakes need adjusting at the wheels or there is air in the system. When those Huck brakes are correct the pedal just barely goes down. You just apply pressure to them.
George

[starterman99]

check to make sure the vent in master cylinder cap is not plugged up and building pressure as fluid heats up.

[mr48chev]

Fluid heating up should not have any bearing on what he is experiencing but a plugged vent cap would cause an issue with fluid returning to the reservoir as there is no place for the air to go.

Just enough clearance on the pushrod so it lets the piston go all the way to the back of the cylinder is enough, Maybe 1/16 as if you can see the rod move at all before it contacts the piston you are good. That is at the piston.

The Huck brakes are a bit tricky as you have to adjust the shoes individually.

[dsraven]

if your truck has a few miles on it and may be showing some wear and tear on linkages etc I would suggest to start at the pedal and work your way through all the linkage until you get to the master cylinder. each pivot or joint should be free moving but not sloppy. excessive play in the joints will cause you to lose pedal travel to make up for the play in the joints. there should be some sort of pedal bump stop (usually on the pedal lever outside the cab so it would bump against the firewall near the brake light switch) so the pedal returns to the same height every time and also a pedal return spring to overcome the weight of the pedal and keep the brakes from being applied as the pedal bounces when driving over rough roads etc. ensure the linkages are all tight but free moving, lubed and not binding on anything. check that the master cylinder is also returning fully and the piston is not hanging up in the bore. you may need to pull back the rubber boot around the pushrod to see that. also look for wetness in that area as that would indicate a leaking master cylinder. check the brake lines for kinks, dents or mechanical or rust damages. check the brake hoses for cracks, soft spots or leakage. a damp spot from a small leak will usually keep dust and dirt stuck to it so beware of that when checking the lines. check the condition of the brake fluid as it is prone to attract moisture which will cause rust/corrosion in the system. that is why it is recommended to change the brake fluid fairly often or at least at every brake job. check the bleeder screws as well to ensure they are not leaking or plugged with dirt. when you have done all that then adjust the pushrod for the master cylinder, ensuring the pedal is seated at the top of it's stroke, and then ensure you have free play at the pedal. next check the park brake mechanism to ensure it is also free moving and not binding anywhere. some guys will say they don't need one but actually having a park brake is part of the brake adjustment and without it the brake shoes won't fit the drums properly. once the park brake is checked and found operational then adjust the park brake to be loose. I know it seems counter intuitive but it needs to be released and loose so the service brakes can be adjusted properly. now adjust the service brakes by turning the star adjuster wheels so the brakes are fully on and the star wheel is hard to turn. now back the star wheel off a few clicks until the wheels can be turned again without the brakes dragging. now adjust the park brake.
it can be a good idea to remove the drums and check things out to ensure everything is as it should be. be aware that the brake dust is a carcinogen. I usually wear a particulate mask and once the drum is off I spray the area down with a garden hose or a pump sprayer with water. this does 2 things, it gets rid of the airborne dust and it also cleans the brakes off. check to ensure a friction material hasn't become unbonded or un-rivetted from the shoe causing issues with the lining binding against the drum. also check for cracked linings. another common thing is the backing plates getting worn and grooved in the area where the brake shoes rub the backing plates. a groove will allow the shoe to "fall in" to the groove and not want to come back to the correct position when the brakes are released. check for broken or stretched springs, broken or stretched shoe retainer hardware, seized or broken park brake components, leaking or sticky wheel cylinders (pull the rubber boot back and check), seized adjuster mechanisms, grooved out brake drums, leaking axle seals etc.
adjustment:
the star wheel adjuster will set the brake shoes to fit that end of the brake shoes against the drum, the opposite end of the shoes from the wheel cylinder. the other end of the shoes, at the wheel cylinder end, will be retracted back into the wheel cylinder by the shoe return springs when the brakes are released. that is where the park brake adjustment comes into play. the park brake mechanism will adjust the wheel cylinder end of the shoes to hold a limit for how far the shoes are allowed to return and push the wheel cylinders back into their bores. if the park brake is not adjusted correctly, or it is missing entirely, then that end of the brake shoes will be drawn all the way back when the brakes are released, with no adjustable stopper like it would have if the park brakes were set up right. so that means, when the service brakes are applied, it will take a lot more brake fluid to be displaced by the master cylinder in order to take up the slack and push the wheel cylinder pistons and the shoes out against the drums. the brake pedal travel will be excessive so the brake pedal will be low in order to get brakes or the pedal may even need to be "pumped up" to get the brakes to work and especially if the other end of the shoes, at the star wheel, is also out of adjustment. back in the day when guys would say "you gotta pump the brakes" it was usually because the brakes needed adjustment and the park brake didn't work. the brakes needed to be pumped in order to move enough brake fluid to get the shoes out against the drums. then the brakes would work. they would be the guys who used a rock as a park brake.
when the brakes are all adjusted step on the pedal hard and hold it for a minute. the pedal should stay at the same height for the entire time you are holding it. if the pedal slowly drops towards the floor, and there is no external leaks in the system, then you may have a master cylinder that is leaking internally. it is allowing brake fluid to leak past the internal piston seals back into the reservoir. I know it is really hard to see on these old master cylinders, located under the floor and with a small lid, but when you step on the brake pedal there should be a small fountain of fluid or at least a ripple of fluid in the reservoir. that is because for the very first part of the brake pedal travel the piston inside the master cylinder is behind the reservoir to bore hole. the hole that feeds fluid into the bore and also allows fluid to return to the reservoir when the brakes are released. when the pedal is pushed the piston actually squirts a bit a fluid back into the reservoir before the piston goes past the hole and the fluid is trapped in the bore and gets pushed out to the brake system. you can tell if the piston is returning fully if you can see the fountain or the ripple when the brakes are applied. pretty hard on these trucks to see that though. a firewall mounted master cylinder is way easier to see this.
make sense? hopefully it didn't confuse anybody

[dsraven]

the pedal pivot under the floor in front of the master cylinder also be a problem on these trucks. it gets dirty and gummed up so it sticks and may be causing the master cylinder pushrod problem you are having. there is a coiled spring on the pivot shaft to help return the pedal but if the shaft is dirty and/or gummed up it may be too much for the spring. you may wanna try taking it apart, cleaning the parts, checking for wear etc and if all is good lube it all and assemble. really, if nothing changed from before when the brakes worked properly, the pushrod length shouldn't need to be messed with. the problem will likely be something sticking, binding or worn out. check the cab mounts as well because if the cab has moved somehow then the pedal, which bumps against the cab with a rubber bumper but is attached to the frame, will also be "different" or "changed" which can have an effect on the master cylinder pushrod adjustment if the pedal doesn't come back to the same place as before the cab "moved".

[jeffbcn]

Since I have adjusted the master cylinder rod for free play I have not had the pressure build-up return the pedal is a little low but it probably needs to be bled and hopefully not adjusted thank you for all the replies they were very informative