Brake problem

I have a 79 jeep cj 5. We have a new master cylinder and new brakes in the front and new lines. After a few minutes of driving the brakes totally lock up and it becomes almost unmovable. Any one else ever had this problem?? We don't know where to go next to fix it. Any suggestions would be greatly appreciated. Thank you in advance.

Dawn & Goerge in NY

Your master cyl is bad. There is a valve in it which is not releasing the pressure in the front lines when you release the pedal.
Chances are your master is not NEW but REBUILT.
Rebuilt masters are notorious for being troublesome, regardless of brand.

MagicRat is right, there might be a “residual pressure check valve” in the master cylinder port that you connected to the front brakes. Normally this is in the line to the rear drum brakes but sometimes things get mixed.

As likely as this is the cause, the first check to make is for brake pedal free play. With a power brake system this is likely not a problem but check anyway. You need 1-2 inches at the pedal pad.

Second check is to take off the master cylinder cover and have someone step on the brake pedal while you watch from a safe distance. What you are looking for is a swirl (gyser or spirt) from the reservoir when the pedal is pushed so protect the paint.

If there is a swirl (gyser or spirt), then it likely is the valve above.

If no swirl (gyser or spirt), then you need to check the placement of the pushrod from the booster to the mastercylinder or a small rubber disc inside the booster where the pushrod sits.

If these are incorrect, the piston is held forward in the bore and blocks a small port in the bottom of the reservoir that allows the fluid to expand when heated without applying the brakes. Blocked, it traps the heated fluid that expands and the brakes lockup after it is driven for aways.

One other cause is a deteriorated brake hose where the inside makes a flap into a oneway check valve that does the same things as I described above – but you said you changed the hoses???:smile:

Hope this helps.

JD

Thank you both very much. We will try these very soon. Appreciate the help.

I have some questions about the brake systems on jeeps... dont want to seem like a total idiot.... my fiance owns a 1989 Jeep Wagoneer... and the brakes have been nothing but a problem... yesterday.. him and i bled the brakes after replacing a leaking break hose...it is making this really loud winning sound and hesitates when breaks are applied?it was making the sound before we started working on it... we think its the compressor... not quite sure.... ? any ideas


sincerely
pulling my hair out over this thing............