I have a 1987 F-700 with lucas girling brakes the mechanicle part has been rebuilt, front calipers and pads, rear shoes and drums, new master cylender but after driving for a short time the front brakes start to bind and eventualy wont release at all untill the truck has sat for hours. I have noticed that with the engine running and the park brake released there is a constant knocking sound that i have traced down to the hydraulic line going from the hydroboost to the park brake control valve it pulsates kind of like a heart beat but I cant determine what is causing this. Any help out there??

 

Thread moved to the "Large Trucks" forum - and WELCOME to FTE!

 

Thanks Krewat

 

Is it possible that you have a pinched line anywhere? Not allowing for free flow in the system? I know that you replaced all the brake components, but as I have always said, every part is made by man and can fail, or be bad right out of the box. You might have a bad master cyl right out of the box, not allowing the fluid to return to the reservoir, but it sure sounds like an obstruction in the system. The sound of the heart beat MIGHT be air trapped in the hydroboost system.

 

For what this sounds like there are 2 possibilitys.

1 has there been any new exh work or new brake lines run lately?
I have seen this and it is rare but it does happen.
The brake line get heated up by exh heat causeing the brake fluid to expand and create pressure. this will cause a very hard pedal when the pedal is all the way up.

2 the rubber brake hoses are bad. rotten on the inside. with age they can get soft on the inside restricting the passage till it is almost a leaky check valve sort of situation.
I've had brakes dragging on my 99 Jeep Wrangler swapped caliper thinking that was it. shamefuly it was not, it was the hoses.
this one is more common than you might think.

 

Thanks GM it does act like its boiling the fluid and the front wheels seem to be the sorce of the heat, I have allready ordered new hoses for the front and hope that solves the problem, but I still haven't figured out what is causing the pulsating hydraulic line between the hydroboost and the control valve. any thoughts on that?

 

While it's possible that both front brake hoses are bad, the other thing that would cause this issue with a new master cylinder is that the pushrod to it needs to be adjusted. If it is slightly forward or the master piston is slightly more to the rear the piston(s) will not fully retract and the compensating port will to open allowing for full fluid to the reservoir. You would be running a closed system and caliper heat would expand the brake fluid and keep the front calipers applied.

One way to check but not as a normal operating condition is to back off the master cylinder nuts when the front calipers are binding and see if they release. A drive around an empty parking lot would be sufficient.

Can't help on the pulsation. It's been 30 years since I worked on this god awful system.

 

Id think if there is a check valve on the line it could be bad or the check valve could be in the back of the pump its self.
Thats my best guess

 

Thanks Guys, all your responces have been very helpful I'm going to take all this info and go fight this giant one more time. wish me luck! PS fmtrvt is right this is "one auful brake system" Thanks Again Ill post my results.