Brake issues

[Kevin 007]

Ok so the other day in my 95 F350, DRW, 7.3, the brakes started to sink to the floor when the truck was running, as I was trying to maintain a steady hold on a hill, waiting for a traffic light. No leaks in system, including the wheel cylinders, so I assumed that that master cylinder was bad, being original. So I replace it along with the original front calipers. I bench bled the master by plugging the outlet ports and pumped till the bubbles stopped in the reservoir. As I was gravity bleeding the entire system for a couple hours, I adjusted the rear brakes until I got some drag when spinning the wheels. I then started bleeding the RR on its own, into a jar at first and then holding the pedal down and cracking the bleeder, moved to the LR, then the RABS, then FR, the FL and then I cracked each fitting at the master cylinder while having the pedal held down, just for good measure. I have done this 4 times now, gone through 3 quarts of DOT 3. I pump the pedal probably 10 times before cracking any line. Im not seeing any air escaping from anywhere, just solid fluid.
And my pedal still sinks, worse then before. If I make a hard application quickly, it will hold hard for a second or 2, then start sinking. This is with the truck running, and with it off (no vacuum); it holds and doesn't go down, so the new master should be good right?

Any thoughts?

 

[greenskeeper]

how's the vacuum pump?

 

[Kevin 007]

I have not put a gauge on it but with the engine off, the pedal firms right up and does not sink no matter how long I hold it, with the engine running, it sucks the pedal right down to the floor, so im thinking my vacuum pump is producing adequate vacuum.

 

[Kevin 007]

Is there a way to de-activate the RABS without bypassing it to see if its the culprit? I don't have a section of line to bypass it at the moment and im a long ways from town

 

[79jasper]

Thats how mine acts when the rears are way out of adjustment.
You can take the nut off the end of the valve and remove the spring.

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[Kevin 007]

Ok I might do that, I unplugged the wiring connecter on the back unit, that didn't do anything

 

[79jasper]

It's either remove the spring or the whole piston.

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[OLDBULL8]

Brake problems are sometimes a ***** to diagnose which you seem to have.
Do you have a dual MC reservoir or a single reservoir?
With a dual reservoir, bleeding the MC takes a special way to bleed.

1st test, Apply the parking brake foot peddle just enough to take up slack. With key OFF, apply the brakes (no pumping), note how far peddle goes down for solid peddle.

2nd test, Apply the parking brake foot peddle just enough to take up slack. With key On Engine OFF, apply the brakes (no pumping), note how far peddle goes down for solid peddle. Is it the same amount as with key OFF?

3rd test, Apply the parking brake foot peddle just enough to take up slack. With key ON Engine running, apply the brakes (no pumping), note how far peddle goes down for solid peddle. Is it the same amount as with key OFF? Does the brake peddle sink to floor? If it don't sink to floor, rear brakes need adjusting.

4th test, if in 3rd test, if brake peddle sinks to floor, remove the RABS large plug, remove spring, leave piston in replace plug, with engine running, apply brakes 3-4 times, replace spring, with engine OFF, apply brakes and hold peddle down, bleed the RABS, repeat twice.

If the rear brakes didn't need adjusting, release the parking brake.

Do you have a solid brake peddle now with engine running?

Make sure you have checked the rear wheels for brake fluid running down the tires, very hard to see sometimes.
Answer questions, then we'll go from there.

 

[Kevin 007]

Ok so with the spring removed from the RABS valve, the brakes are working pretty good again. If the engine is running and I really apply firm pressure to the brakes, they will still slowly sink to the floor, but this would be more pressure then I would need to stop and hold the truck. Thanks for all your support fellas. With new calipers, wheel cylinders, mater and a couple break lines, and fresh fluid throughout, the trucks breaks are feeling pretty good, certainly much better then they were.

 

[compressionignitionrules]

reading through this I would have said the replacement master was your issue or lots of air in your brake system after some strange bleeding practises. I've replaced bad master cylinders before with only bleeding at the master, but very slow brake applications and only after hose bleeding back into the reservoir on the bench.

Living in BC you need those brakes working right! in this part of Ontario I can run on ****** brakes for a long time, flatland and always a safety shoulder on 2 lane roads