Fading Brakes

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Jul 10, 2009 | 03:34 PM

starweb

Thread Starter

Senior User

Joined: Jul 2001

Posts: 155

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From: Omaha, TX.

Fading Brakes

My brakes are acting up. one second I hit the brakes and they are nice and firm. next time they go almost to the floor! Then I pump them and they are fine. Is this the Booster or the Master Cylinder?? Master cylinder is full of fluid. no leaks.

Jul 10, 2009 | 04:07 PM

a truck named effy

Mountain Pass

Joined: May 2009

Posts: 232

Likes: 1

From: 40min from Charlotte NPD.

If you are not loosing fluid it sounds like a bad seal in the master cylinder to me. The fluid is staying there it's just that the piston is moving past the fluid rather than pushing it. However there are more knowledgeable people on here than myself and I all ways wait for at least 4 responses before I make and conclusions.

Good Luck

Jul 10, 2009 | 06:41 PM

fmc400

MSEE

Joined: Apr 2004

Posts: 10,386

Likes: 35

From: Austin, TX

There's air in the system; you could try bleeding the brake lines to see what that does. If the problem keeps happening, it's probably the MC as effy said. Soft brakes is almost always air in the system. The softness comes from having to compress the air bubbles to get the fluid to actually start moving. Air-free brake fluid itself can't be compressed; that's why it's used as a hyrdaulic fluid.

Jul 10, 2009 | 09:07 PM

4x4 Bart

Fleet Mechanic

Joined: Feb 2007

Posts: 1,326

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From: Middle TN

Need new master cylinder, seals are too worn. Effy nailed it.

Jul 11, 2009 | 05:59 PM

starweb

Thread Starter

Senior User

Joined: Jul 2001

Posts: 155

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From: Omaha, TX.

Ok, so, I put in a new master cylinder. Had to do some serious bleeding just to get the fronts working. Could not get the backs to bleed at all. DAMN IT JIM!! Disconnected one of the lines at the Wheel Cylinder in the back and pressed the brakes. Nothing, barely a drip. FINE THEN!! AAARRRGGGHHH!! Put it all back together and limped it up to Les Schwab. Three hours later the guy at Les Schwab calls me. He said, " It never fails, every saturday I get something in here that drives me nuts. However, I figured it out. There is a valve near the front that the lines run thru. It is worn out and plugged the lines to the rear. Will have to wait until Monday to get one."
Good, I knew it had to be some obscure part causing the problem. I've owned too many of these trucks and I knew that I was doing everything right.

Jul 11, 2009 | 07:08 PM

fmc400

MSEE

Joined: Apr 2004

Posts: 10,386

Likes: 35

From: Austin, TX

That "obscure part" is the proportioning valve and that's a little odd it'd need replacing. You did bench-bleed the MC before you installed it, correct? With a vice and all?

Jul 11, 2009 | 08:24 PM

shortbed1

Senior User

Joined: Jan 2003

Posts: 178

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From: beaufort

Hey FMC that is the proportioning valve and i also doubt it needs replacing. He installed that M/C without bleeding it first and there is air in the valve you need to hold the pintle valve in on the valve to get the air to move out then you can bleed the rears. That pintle valve is located right in the front of the valve if you watch the pintle it will move outward when the brake pedal is hit that's because of the air. loosen the line coming out the rear of the valve that goes to the rear brakes hold the pintle valve down and have some one push the brake pedal down easily once pedal is on the floor tighten line and let pedal up this might take a couple of times but it will work.You do not need to replace the valve. Just get the air out. Hope this helps, Chuck