Brakes locking on (1 Viewer)

[JMH]

I have a Manafre front disk kit on my '73 FJ40, and a GM rear disk kit as well. Recently I installed a new master cylinder. It's been working fine until today, when on my way into town I noticed a loss of power. When I took my foot off the gas the car stopped itself. The front brakes were nearly locked; the brake pedal was hard as a rock. I loosened the brake line fitting at the MC to release the pressure and drove home carefully. I tried backing off on the actuating rod to give the pedal more free play, but as soon as I drove it again the same thing happened. Only seems to be affecting the front brakes. Any help would be most appreciated.

 

[beanz2]

Did you adjust the push rod inside the booster? If this is left unadjusted and is too long for the new master, everytime you brake some pressure is left in the hydraulic system and it eventually builds up to stop the vehicle. Imagine pumping the pedal a little and not release it all the way and do it over and over.

The same symptoms can come from frozen front calipers, but I assumed the calipers are new.

Dave

 

[Chef]

If you're running a drum brake master cylinder, you need to pull the residual pressure check valves. Drums require some pressure to stay out against the drums and ready to act, disc's don't...drum masters have a check valve to maintain a small amount of pressure to do this, disc masters don't...

 

[beanz2]

The thing is, Alan, drum brake residual valves at the most hold less than 10 psi, just enough to counter the return springs on drum brakes Though that will make his pads wear out faster, it shouldn't stop his rig.

Dave

 

[JMH]

Thanks for the input everyone. I'll check the pushrod in the booster. When I installed the new master cylinder I was told to pull the rear residual valve but not the front. I've since learned differently and pulled the front one this morning. Now I'll check that pushrod and try it out.