Brake Puzzle... Genius required...

OK, did a 4wheel disc brake conversion on Dads SN5. Has chevy discs on Dana 30 front and 44 rear. Brakes work great, dual MC moves plenty of fluid. THey bleed fine, good pedal until...... After about a mile or so of 35-40mph running on the road, the brakes slowly lock up to where no amount of pumping or throttle can overcome it. He has to get out of the jeep, break the bleeder open on the front caliper and relieve the pressure. After this, he is fine. Happens on way to trail, and on way home, never in between. All new lines, new MC, clean fluid, and all new calipers. Any ideas?? Can exhaust routing overheat brake fluid?? Thats my only idea because of the higher RPMs on road. Happens everytime without fail.

Think Tank Members Please.

 

Did you change the residual pressure valve from a 10# drum unit to a 2# unit for disk brakes. The factroy MC will have 10# units installed in the MC, remove them and put Wilwood 2# units in the lines on the outside of the MC, 1 for the fronts and 1 for the rears.
In the picture it is the litle red thing attached to the brake line.

 

The MC was a dual reservoir from herm, Im not sure what valves are in there, but do you think thats a cause for the lock up, it doesnt make sense to work for so long offroad, and then lock up at onroad speeds.

 

Your problem is a symptom of a plugged compensating port on an unmodified vehicle. The vent (compensating) port allows for the expansion of brake fluid when it gets warm. If the fluid gets warm or hot with no way to compensate for expansion, the brakes will start to self apply, sometimes to the point of bringing the vehicle to a complete stop. I think this might be the case since they only start to drag when driven at higher speeds.

If you take the lid off of the master cyinder and sllooowwly press and release the brake pedal, do you see any fluid movement in the reservoir?

Also, can you back off the adjustment on the linkage for the master cylinder?

If the sealing cup inside the master cylinder is positioned incorrectly, it could have the compensating port sealed off when at rest, causing the condition you describe.

Though I don't think it's the cause of your problem, do like Mike says and replace the factory residual pressure valves. They can cause the discs to drag though they shouldn't cause them to completely lock up....

Hope this helps. Let us know what you find!

 

That sounds like a much more likely scenario. Mruta, if I do see fluid moving, what does this mean, and how would I realign, or remedy the problem. I am not near the vehicle now, but could you give me an idea of what would be involved with repairing it?? thanks in advance, I think I will explore this route. My only question is, how does this valve stay closed when the brakes are intentionally applied, increasing line pressure? Any ideas??

 

PS, had a girlfriend in college that was from Lisle. Great little town. How long have you been there??

 

I had this problem on my CJ2a, rust was blocking the compensating port.
Check the pedal or rod adjustment to the master clyinder. If your adjustment won't let the rod on the master clyinder come back all the way, the compensating port will be blocked.

 

I've lived in Lisle for 3 years now though I've lived within a couple miles of here my entire life (except for college). What was her name? Maybe I know her...:twisted:



Back to the brakes, one of two things will cause the compensating port to be blocked:

1) Dirt, rust or other crap is blocking it.
2) The linkage/pushrod is adjusted too far, not allowing the piston to retract fully and causing it to block the compensating port.

Let us know how it goes.

 

Her name is Christine Jones, her father Kelly was a Engineer for GM's Electromotive Division in a nearby town, big rock quarrys nearby I think. Great Family, Great little burb. Couldnt handle the traffic for 1 week.