Brake pedal

[klutz51]

I have a s-10 chassis, under cab brakes, brake pedal pumps up but looses pressure. It is cpp master disc/disc set up with 2lb valves, no leaks. Has anyone run into this or do I have a bad master?

[dsraven]

if you pump the pedal up and it gets firm but slowly bleeds down as you hold a constant pressure on the pedal, it sounds like the master cylinder is leaking internally.
check the linkages to ensure, firstly, that all the pivots and connections are nice and tight, no slop that can affect your total master cylinder piston travel, secondly to ensure your pedal is returning to the top of it's travel and also giving the master cylinder a full stroke. there should be some freeplay at the top of the pedal stroke and then if a full stroke is made, the pedal should go not quite to the floor before the master cyl bottoms out at the end of it's stroke. also check the power brake booster pushrod adjustment to ensure it isn't too long. if it is too long then the master cylinder has trouble getting fluid from the reservoir. I say this because if you have all the adjustments and linkages correct and you have a disc/disc set up then you shouldn't have to pump up the pedal. usually the pedal pumping is due to the drum brakes being out of adjustment so the pedal needs to be pumped to bring the brake shoes out against the drums, then the pedal should be firm. with a disc/disc set up the pedal should be firm in the first 1/3 of a pedal stroke because the brake pads are really quite close to the discs so not much fluid is required to be displaced. I would check the linkages and the master cylinder bore size to ensure it is the correct one for your set up. really though, if the pedal bleeds down slowly with a held brake application, similar to holding the brakes at a traffic light, and there is no leaks externally, then the master must be bypassing.

[klutz51]

Its mot bleeding down once pumped up and holding it. You pump it to a firm pedal and it holds until you let off for a few seconds and then it goes back to the floor. I have a remote resivoir and you can see the fluid go down when pumped and fill the resivoir back up when let off.

[mr48chev]

That says you still have air trapped in the system somewhere. The pedal dropping but being able to pump it up and hold pressure.

As much as I hate seeing those ugly boosters and master cylinders on AD and TF truck firewalls we kind of pick our own battle when we put the master cylinder and booster under the floor on these trucks.

Do you have the proper residual valves in the brake lines? This is a must have or you end up experiencing this symptom. If not the fluid wants to flow back to the reservoir rather than stay in the line when you let up on the pedal. Most drum/drum master cylinders have residual valves inside the the fitting where the line fastens to but disk master cylinders don't.

Not trying to pick on you or imply but having worked on brakes daily as part of my job for a number of years when I was working as a front end and brake mechanic ( When Tech was Texas Tech and not a guy who worked on cars) I found that a lot of "we replaced our brakes and now they don't work right can you fix them?" deals were guys who didn't bleed the brakes properly.

A few issues there are:
helpers who pump the pedal too fast which doesn't allow the fluid to flow from the reservoir to the cylinder right. They have to pump slow and steady.
Helpers who let off the pedal the minute it goes to the floor before you close the bleeder.
Running out of brake fluid in the reservoir. Had a buddy trying to bleed the brakes on his car who did that about three or four times before he called me. he was amazed that I refilled the reservoir after bleeding each wheel and kept it full.
Trying to bleed the brakes yourself without a helper by having a hose off the bleeder running into a bottle with some fluid in it as air sucks in around the bleeder screw even with the end of the hose in the fluid.

You can make an inexpensive pressure bleeder out of a 10.00 weed sprayer from the hardware store. This is one of the simpler and not elaborate or spendy DIY pages on making one. You have to figure out a cap for your reservoir setup but that would be the main difference from what he is showing. Several of the similar instructions show putting a low pressure gauge on the sprayer rather than the tire valve he shows. That would be personal choice but you don't want over 20 lbs of pressure in the tank and only need about 15 for it to work right. http://faculty.ccp.edu/faculty/dreed...eder/index.htm
I'm not sure that you would be able to make something like that work with your setup but it is a thought to ponder.

[dsraven]

another thing to check is maybe you have a master cylinder that has the residual valves in the master cyl as well? some rebuilt cylinders used to be a disc drum set up and have the res valves in them still, sold as disc disc after the rebuild.
still, ensure you have free play and full stroke of the master and also no play in the linkage that will take away from a full stroke or leave you with no pedal freeplay
like said, bleed it out again, ensuring that you have no air in the system. start at the wheel furthest away from the master, then the next closest, and so on. usually the passenger's side rear, then the driver's side rear, then the pass front and finally the driver's front.a a vacuum bleeder drawing from the bleeder screw is another idea which you can do yourself once you have the vacuum part of it. pretty cheap these days at places like harbour freight. a one man bleeder is also easy to make it's just hard to see if you have the air out