Brake question....at my wits end

I installed a new Master cylinder on my 77 cj 5 with Disc front, and drum rear manual brakes (stock setup). I have bled them thoroughly (as far as I can tell) and the master cylinder is fine.

Problem is: After about 15 minutes of driving, when the motor and underhood area is hot, the pedal tops out, stays rock hard, and the brakes drag BAD. So bad I cannot drive it. This only happens when it is HOT. Right out of the garage, it is totally fine and normal. What the heck is going on???? Any ideas?????

 

any brake lines passing too close to exhaust/headers?

 

NO. I have not changed anything from the way it was before this happened. I only replaced the old master cyl. because it was leaking. That is all I replaced, and now this problem. Worked fine before!:? :? :?

Note: I am using prestone sythetic DOT 3 fluid.

 

Did you put in a master cylinder for a Drum Drum setup? If so you need to removed the residual valve and put in one for disks.

 

Do you have a TSM? There are 13 pages of diagnosis charts in the TSM - way too much to list here.

Did you hold in the metering section valve stem on the combination valve when you bled the brakes? Air in the system should give you a brake warning light. Could also be a binding bellcrank pivot on the MC, adjustment of the pushrod, improper fluid, etc.

 

Used the STOCK pushrod setup. IT is the correct MC for the front disc, rear drum. Bench-bled the MC completely.

No brake warning light....if it even works.......can't say that I know what you are talking about, TIMGR. Forgive my newbie ignorance.

How could the fluid be improper?? (listed in previous post...) The rod NOT binding...... Again: the whole system works just as it should until the engine is hot, then THIS PROBLEM. What the heck?????

This is killing me

 

Maybe this will help
http://www.piratejack.net/troubleshooting/Main.html

I am thinking maybe a tripped combination valve?

 

I have had two old jeeps, one a 53 M38a1 and the other a 56 CJ5 that had their front brakes dragging just enough to keep if from performing in 3 rd gear. A couple of quick ways to check your brakes is one to feel of the wheels with your hand after driving your jeep. Or stop on an incline and see if it will roll.

In my case the 2 short rubber hoses to the brakes on the front wheels were not letting brake fluid return to the master cylinder . They were old and not allowing the brake fluid return passage to the m/c.

Just like your jeep, the first time I would drive it in the morning, it ran great. But as soon as the brake fluid built up pressure and not letting the brake fluid return it would build up pressure and not let the front brakes release. If they are old, you might want to consider a trip to the parts store and put some new ones on. I bet this is your problem!

 

I will try those suggestions this week. Thanks for all the helpful tips, guys! Much appreciated. I will check back if you all have any other suggestions. Thanks again.

 

Your Jeep has a combination valve that meters the brake pressure between the disks and drums, and also has the brake warning light switch. It's mounted on the frame rail. You can test the brake warning light by pulling the connector and grounding it. You should also see the same warning light if you try to drive with the parking brake on.

 

Thanks for the clarification, Tim. I will check it out. Thanks again, guys.

 

I am going to try a different master cyl. Now that I have talked to you guys and several friends, I think this one is bad. Not letting the fluid return. Damn if it is a new MCyl, also-- not even a Remanufactured one sheesh!!

 

If you already have a new Master cylinder on it. Before doing it again, check those short rubber hoses from your metal brake line to the brakes on your front wheels. I bet those are your culperts!

 

Sounds like it could be another case of a blocked vent port inside the master cylinder... seems to be a lot of this going around lately.

Take the lid off of the master cylinder, have a helper slowly press and release the brake pedal. You should see some fluid movement in the reservoir.

Do you?

Also, can you tell which axle or wheel is/are dragging?

 

x's 2. I had this same problem... Exact same symptoms - it would do fine when you first started out, but the longer your drove, the stiffer the pedal got. It also held my brake lights on. I would have to open one of the front bleeder screws to release the pressure when I got somewhere.

 

SO you guys think it is my front hoses? It seems to drag ALL 4 WHEELS evenly when it does this. About 15 minutes into the trip. Never did it before the new master cyl. install, and now it does it!

What about these front hoses? Blocked????? It bleeds well and evenly when I bled the brakes..........

Still wondering.

 

I think it's the M/C. See my above post and let us know what you find (too lazy to type it all again).

Good luck!

 

If the problem didn't start until you replaced the M/C, that is where I would start.

 

Make sure that the push rod isn't adjusted too long. I think this was stated before, but if too long it may not allow piston to uncover return ports.