Brakes Issue...Need Assistance Please

[deucesarewild]

Hi I have read a lot about the braking system on these trucks and would greatly appreciate some help with my issue. I have a 1970 M35a2 that I have been working on and I am stumped on exactly what is causing brake problem. My brake Pedal is high and is like standing on a rock. I have gone over brakes and replaced most of the hard lines due to rust and one was kinked. I have bled the system with a power bleeder and silicone brake fluid has been used. I can barely budge the pedal and it makes no difference whether air pack has air pressure to it or not. Let me know what other info. is needed to diagnose. Any help would be great...THANKS in advance.

 

[Flyingvan911]

Could the master cylinder be plugged up? Could the brake pedal itself be stuck?

 

[73m819]

Is the m/c piston going ALL THE WAY back when the peddle is released, if not the the brake fluid can NOT return to the m/c, if this is the problem, unpin the peddle linkage from the m/c, let the m/c piston seat all the way back then adjust the linkage to the peddle

 

[welldigger]

Does the pedal move at all? Because when I got my truck I had a good firm pedal but I couldn't move it very far. Turns out my air pak was frozen solid. If I had to guess that's gonna be your problem.

 

[m-35tom]

do you have brakes?

 

[deucesarewild]

I will check the pedal linkage although I think the piston is returning all the way back, the brakes work but very poor and I have to stand on them, I have thought air pack problem so far because I can never hear it engage or disengage...to flyingvan911 brake pedal not stuck because moved all the way to floor prior to bleeding and I wasn't aware that master cylinder could be plugged if fluid could be bleed through truck, explain this to me because I haven't thought of this...Thank you all and hopefully with you guys help I can figure this out.

 

[cattlerepairman]

The brake system is a simple, air assisted single circuit system. Using the TM, go through it, step by step, ruling out components. As for the TM, I suggest not to use the 209 series, but 361-20 (brake section includes all components), 361-24 (1 and 2) THIS CONTAINS THE TROUBLESHOOTING GUIDE, and 361-34 (includes brake drums, shoes).

 

[Flyingvan911]

I will check the pedal linkage although I think the piston is returning all the way back, the brakes work but very poor and I have to stand on them, I have thought air pack problem so far because I can never hear it engage or disengage...to flyingvan911 brake pedal not stuck because moved all the way to floor prior to bleeding and I wasn't aware that master cylinder could be plugged if fluid could be bleed through truck, explain this to me because I haven't thought of this...Thank you all and hopefully with you guys help I can figure this out.

Oops. Didn't realize the pedal itself would move.

 

[welldigger]

I will check the pedal linkage although I think the piston is returning all the way back, the brakes work but very poor and I have to stand on them, I have thought air pack problem so far because I can never hear it engage or disengage...to flyingvan911 brake pedal not stuck because moved all the way to floor prior to bleeding and I wasn't aware that master cylinder could be plugged if fluid could be bleed through truck, explain this to me because I haven't thought of this...Thank you all and hopefully with you guys help I can figure this out.

Air pak is frozen.

 

[DeucesWild11]

I'm thinking your Air Pack is frozen too..

 

[deucesarewild]

Ok thanks, I have always thought air pack problem but didn't want to assume without some others knowledge or experience with this issue. I will pull the unit and commence to check and rebuild...wish me luck and THANKS again

 

[welldigger]

Pull the air pak and pull apart BEFORE you buy a rebuild kit. Sometimes they are too far gone to rebuild.

 

[painter paul]

If the air pack shaft gets rusty inside it can stuck when the brakes are applied and not return. I had a guy road test a 5 ton dump and slam on the brakes hard and they locked up on the highway. Had to bleed the pressure off to move it. The shaft can also stick and the brake pedal will keep getting higher up with each application til the brakes stay on. Again, have to bleed the pressure off to get the truck to move. I was able to take the pack apart and clean up the shaft w/o a kit and it worked fine. Thanks, Paul

 

[deucesarewild]

Ok good to know, I tried to oil it and cross my fingers that it would free up. (welldigger)---thanks for the input I will definitely inspect before spending money on a kit...Thanks

 

[deucesarewild]

Removed Air pack this afternoon and disassembled...it was frozen stuck, too much build up and rust not to go ahead and hone and rebuild, plus all gaskets and seals look bad. So any recommendations on a particular kit to buy or not buy... I guess I mean quality wise? Chinese made vs. American made? And again thanks to all for pointing me in the right direction.

 

[welldigger]

Which style do you have? Short or long? The rebuild kits do NOT interchange. The long kits are pretty easy to come by. The short ones are a little harder to find but they are out there. Also one other thing how deep is the pitting on the air end? Try honing it before you buy the rebuild kit. The cylinder needs to be smooth otherwise air can bypass the piston and chew up the rubber swab around the piston.

 

[deucesarewild]

It is the long style for sure. Pitting looked bad until I lightly used a wire brush to remove the excess gunk and rust, doesn't look too bad...never used a honing kit before do I just use until smooth inside or is it possible to remove too much metal?

 

[welldigger]

To hone just get an engine cylinder hone available at any auto parts store. Generously lube the inside of the cylinder with a light oil. Chuck your hone up in a hand drill and insert it in the cylinder. Pull trigger and smoothly and steadily move your hone back and forth the full length of the bore. Stop and check progress frequently. I suppose you could remove too much metal but you would be there a long time to get to that point. You only want to remove just enough metal that the inside is smooth. Its one of those use your own judgement and common sense things. Once your happy with the results clean the crap out of it with brake cleaner.

 

[gungearz]

Don't you think a brake cylinder hone would work a lot better...?

 

[swbradley1]

If you could find one large enough.