Humvee Brakes Locking Up

[jkupka]

Need some help. My 87 Humvee is sorta back on the road but with a major problem. After picking it up in December, its finally almost restored. However, after doing a small road test my brakes are locking up. I'm building way too much brake pressure and all 4 disc are locked to the point that it wont move. I have to open the bleeders and fluid shoots about a foot out of them. Then I'm able to move it and then again after hitting the pedal numerous times it begins to slow down and lock up again. It has DOT 5. The weird thing is that I drove it back and forth to the body shop before all this and nothing happened but I might not have been so hard on the brakes as it was only 1/2 mile each way and then it sat for days each way. Thoughts?? I bled the system again as well. So, either hydro booster is bad or v belts need tightened or ??? HELP>>>>

 

[dilvoy]

You must have done some work to the hydroboost or the brake master cylinder or both, because what you have is a master cylinder piston that is not returning far enough for the fluid in the brake lines to re enter the master cylinder after you let off the brakes. Might be a bunch of crud in the master cylinder causing this, but they you would probably have brake fluid leaking out the back of the piston bore too and maybe into the hydrobooster and or out onto the ground. More likely, you changed the master cylinder and there is something wrong with the compatibility of the back of the piston and the rod that activates it that is in the hydroboost. There is sometimes and adjustment on the end of that rod, but before you start messing around, Did you change the master cylinder and if so, where did you get it from? Of you have an USMC Humvee you should have an O ring at the back end of the master cylinder so you can go deep fording without water and dirt getting into the back end of the master cylinder bore and into the hydrobooster. If Army or Airforce, they don't have the O ring, but you can install a master cylinder that has one without problem. Did you happen to buy a rebuilt master cylinder? ZTo be cheap, you could make a shim of the distance you need and place it between the master cylinder and hydrobooster and make things work normally again.

 

[Action]

Is it possible to mate a hydrobooster and master cylinder that aren' supposed to be together?
My old VW did this once, when the inside of a rubber line collapsed. The pedl pushed it thru one way but didnt come back on it's own.

Is the pedal coming all the way back up after you let go?

 

[dilvoy]

Few Humvees in civilian hands have rubber brake lines and yes, many master cylinders will bolt onto a hydroboost. The adjustment of the rod in relation to the socked that is in the back of the piston assy is critical. Yes you can mix and match parts, but you need to know what makes the system operate normally before you go messing around and swapping parts on purpose. Most often it is a wrong part installed for some reason. Wrong part in the box from a parts outlet that takes returns, incorrect part for the application too. You can sometimes notice a different shape to the Brake Pedal end of a Hydroboost rod system. Sometimes round, sometimes pyramid shaped, then there is the master cylinder end too.

 

[jkupka]

Hey guys. Here is some more info. I didn't remove the hydro booster or master cylinder. The only thing I did was drain the old fluid and refill with DOT5. I line vacuumed each fitting to remove the old fluid till the new appeared. I'm going to adjust my V belt and do the booster test that I found. Like I said all 4 disc lock up at the same time, so I would assume the proportioning valve is ok as its all 4 and not just one set.

http://www.hummerknowledgebase.com/brakes/hydroboost.pdf

 

[Action]

Dont vaccuum DOT 5.

What was in it? So you just poured new fluid on top of old?

 

[jkupka]

no......I opened all the fittings and let it trickle out and the used a hand vac bleeder.....is that my issue? it was old yellow fluid and about empty

 

[911joeblow]

This sounds like classic tight pedal to me. The pedal needs free play before it engages the Master so that as the fluid heats there is room for the expansion. The pedal free play is what allows for expansion without the brakes binding up.

If the brakes were weak or had contaminated fluid with air in the system a mechanic may have tightened up the free play to overcome the soft pedal, by you fixing the issue by bleeding the system and putting in new fluid you need the free play back again.

 

[Shoprat 19th Engr]

Check the linkage off the pedal it should move freely about a 1/4 inch if it does not it is not releasing all of the pressure off the master cylinder and it will build up pressure so that will make the brakes lock up

 

[Recovry4x4]

Since all the brakes are locking up, it is the classic symptoms of a blocked compensating port. As a troubleshooting technique, you could shim out the MC with a couple of washers and do a test run.

 

[jkupka]

Its troubleshoots as a faulty hydro booster. The pedal has never been touched and no issues after road testing it several times before I messed with the fluid. Something has blocked the check valve. Fluid surface should have some movement or spout in the forward reservoir section. Minor spouting may occur in the rearward reservoir. If no movement or spouting of fluid occurs in forward reservoir, the hydro-boost unit is defective and must be replaced or repaired. I had zero bubbles and the more I pressed the pedal, it pushed out the pistons locking all 4 wheels. So the booster is not allow the system to vent properly back thru the power steering.