68 cutlass converted from manual drums to power and front disk. All new lines, proportioning valve, booster, master cylinder and everything from inline tube. Already replaced prop valve once cause it was leaking from the rear "metering valve@ or whatever that is that has the rubber cover over it. The main prob is after I drive the car for a bit the pedal gets rock hard. Last time it did it I jacked up the car and I couldn't spin front wheels. Cracked the but at the master to relieve pressure and wheels spun no prob. What would cause the pressure to not be relieved. Very strange. Already replaced brand new booster because friend put the long push rod in by mistake, jammed on pedal when bleeding and you heard the whole inside just crack. Pedal feels nice before this all happens after cruising. To long of a vacuum line?
Thanks!!

 

Check the length of the push rod from the pedal to the master.

There is a specification for the clearance between the rod and the master with the pedal up against the stop - something like 1/16 to 1/32 of an inch if I recall. If there is no clearance and the rod is resting on the piston, it'll do this.

- Eric

 

The spec is .010" and as mentioned this is the first thing I would also check. I purchased a MC and Booster from ILT and ran into this spacing issue. The spacer that fits inside the MC was too long and when I put the MC in place, it would not seat against the booster. Tightening the nuts would draw it down but that is not the way it's supposed to be. I ended up grinding down the spacer until the correct space between it and the rod in the booster was achieved. One way to check this is to use a small piece of gum or putty on the end of the spacer in the MC. Install the MC, tighten the nuts and remove it and look to see if the gum or putty shows a gap or if it is completely crushed and displaced out of the way.

 

We ended up putting the other push rid in because yes, the first one was way long and had to tighten nuts just to get it to sit there. Other one let it sit flush. Like I said brakes are fine for 20 mind slowly gets hard as a rock abd front wheels are being help tight. I'll check rod but thinking something else like vacuum maybe

 

Consider the condition of your brake hoses.
If they're old, the insides can swell and create a "check valve" effect, allowing high-pressure in, but nit allowing it back out when you let off the brake.

Check pressures at different points:
If while this is happening you can crack the line at the master and you get a dribble, but you crack the bleeder at the caliper and you get a spray, then that may be the problem.

- Eric

 

Add to the above: faulty master cylinder. There are what's called "compensating ports" in them and if they're not working properly they won't let brake fluid return to master cylinder reservoir when brake pedal is released. They'll keep hydraulic pressure applied to whichever circuit the port has failed on. This problem can be caused by wrong length pushrod as Eric and 69442C have pointed out.

 

I ran into something like this years ago, front brakes being held on. After changing MC, prop valve, booster, we ended up replacing the front flex lines. Worked fine after. Flex lines that were replaced looked ok, no cracks or bulges. Weird.

 

i've seen that brake hose issue too, definitely worth looking at.


bill

 

Ok so I tried something this morning. Been googlibg that crap out of this and a lot of people come down to the pushrod that sits in the booster and pushes the master is to long. Well I think thats it. Put 2 washers inbetween the master and booster and gave the rod a little wiggle room. No locking up, no hard pedal. Just have to bleed and I think Im good. A lot of threads with this issue never have an ending result/answer. Wish people would update there threads when they solve!!

 

HA!

Glad you finally got it sorted!

- Eric

 

Thanks man. Definitely a good feeling not having the car drag and having the front end feeling like its going to melt from the eat. Now I just gotta nail down my timing and see what I can squeeze outta her. I know gears would wake her up but thats not happening right now.