Brake pedal slowly fades to floor?

[unioncreek]

I have a 96 Ram 2500 with the Cummins. I currently have the ABS and BRAKE light on periodically. I have 300K on it and figured it was time to flush the brake system. I replaced the master cylinder and on front caliper(bleed valve stuck), used the one man bleed system. Everything went as planned system bled well. Before changing the master cylinder I noticed that the pedal would slowly fade, but I always had brakes. What would cause it to fade like that after I replaced the master cylinder and flushed the system. I considered a vacuum leak, but I my cruise works and the HVAC will change to any position. Could it still be a small leak and the cruise and HVAC still work. How much vacuum should the system pull and what is the best way to test it.

Bob

 

[crash68]

It's not the brake booster if the pedal is going to the floor, something in the brake hydraulic system is causing it. Did you bench bleed the master cylinder before you installed it?

 

[unioncreek]

Yes, bled the master cylinder first. I pedal doesn't go all the way to the floor. I think I bleed the brakes again to see if there my be some air in there yet.

Update:
Just was out checking the ABS and BRAKE light issue and the brakes are much firmer. I'll drive it for a few days and see if hey get better, but their quite a bit better than what it was.

Bob

 

[unioncreek]

Another update:

Was reading through the service manual about the vacuum pump and that system. Checked mine and it was at 10, on the lower scale. Went out and checked what the vacuum pump what at and it was 27. Through the process of elimination it turns out the line running to the vacuum switch on the fender had a broken line. We'll see tomorrow if it's fixed.

 

[Spike95]

A vacuum problem will cause you to lose power assist on the brake system, not cause the pedal to fade to the floor. It would make the pedal harder to press.

A pedal that fades partially to the floor, and then gets firm, is typically a hydraulic issue in one half of the brake system (either front or rear). An internal leak in the master cylinder will behave in this way. The pushrod going into it will push until the leaking half has bottomed out and then will meet the resistance of the half that is still sealed. It could also be a leak or large amount of air in the system elsewhere causing the same effect. If you don’t see any evidence of a leak, and you are 100% certain that the master cylinder and lines are completely bled, then I would lean toward a failed seal inside the master cylinder. My suggestion is to bleed the system again before condemning the part. Make sure to start at the furthest wheel end from the master cylinder.

 

[unioncreek]

The problem has been solved. After changing the master cylinder and one caliper(couldn't get the bleed valve loose) and bleed the brakes a couple of time. Check the vacuum with the vacuum gauge, turns out it was a cracked rubber connector in the low vacuum switch line.

Bob

 

[Daw14]

Hope it keeps working for you and us .

 

[E7K0629]

A pedal that fades partially to the floor, and then gets firm, is typically a hydraulic issue in one half of the brake system (either front or rear)

Would this happen all the time or could it be intermittant? During normal driving/braking I get a solid bite on the brakes with very little pedal movment. But if I'm driving slow in gridlock or down a hill with very light braking applied for less than a minute, the pedal will sink to the floor and "barely" stop the vehicle. As soon as I let up on the brake, it will return to normal.

 

[18CrewDually]

Would this happen all the time or could it be intermittant? During normal driving/braking I get a solid bite on the brakes with very little pedal movment. But if I'm driving slow in gridlock or down a hill with very light braking applied for less than a minute, the pedal will sink to the floor and "barely" stop the vehicle. As soon as I let up on the brake, it will return to normal.

Usually the cause is debris or corrosion in the master cylinder allowing the seal/valve to pass fluid.