I acquired a retired 90 Ram Van 3500 it was an Ambulance in it's previous life.
This van has rear anti-lock brakes.
I am having brake pedal fade problems. If one gently applies brakes to stop at a light it's fine, but applying a little more pedal pressure while stopped and the pedal fades away...
Making a hasty stop "Not locking up wheels" the pedal fades away again
There are no leaks, the front and rear brakes were relined, master cyl replaced, and brakes bled out, no joy... Problem still there...
Clamping off at the front flex lines, solves the fading issue...
Idea's?

Thank's
Alex

 

That fluid has to be going somewhere. If it isn't leaking out, then it is bypassing some seals somewhere.

 

There is a soft "Hissing" sound when the pedal is fading as well...
I was informed the hiss was the "Anti-lock" adjusting it self.
Could the problem be the Brake Proportioning Valve?
Pretty much the only thing left untouched, other than the Brake booster it self...
Btw, the flex lines are good and not swelling under pressure.

Thank's

 

I had the exact same problem.

I'm guessing there's still air in the line. I had bled my brakes because I was having the exact same issue on my Dodge van. It helped some but the problem persisted.

Then when I was crawling around under the van, I noticed the, "combination valve" (I'm not a mechanic) that I had not bled the first time. So I went back, bled that, then FL, FR, RL, RR wheels and the problem was completely gone.

Other things to check - rear wheel cylinders. See if there is leakage at the rear wheels. Happened with my d250. If it leaks you lose pressure.

Is there fluid coming out of the master cylinder reservoir, or is it wet around the cap?

 

Yes, wet around the cap. when bleeding brakes with cap off fluid was blowing up and out of the MC's cap...

 

wheel cyls are dry, and no leakage accept it's wet around the MC's cap.

 

Fluid blowing out while you are bleeding is normal. If you have the cap off while bleeding, just push the pedal down slowly. Makes less of a mess.

Rear anti-lock is just a dump valve. So, the only thing you should notice when it activates is the pedal going down in 'steps'..... (and a dramatic decrease in braking effectiveness..... hhhhmmmm.....) Unless you have 4WAL, then there is actually a motor/controller hiding under the hood somewhere. It tends to make really odd noises when it's doing its thing.

 

No, just 2w anti-lock

 

just Rear anti-lock

 

it's just a continuous fade till it stops, no stepping.
Clamping off at the front flex lines, solves the fading issue...
What does this bring to mind?