Brake pedal goes to floor only when engine running
Truck is 1992 E350 cube dual wheel cab and chassis
Initial problem - Front brake lines rusted out, lost pressure, MC bled dry.
After replacing all front brake lines and bleeding front and rears, I got good solid
pedal with engine off, I mean I can push long and hard and it stays firm.
When I start engine, the pedal slowly goes to the floor with almost no resistance
and you hear a woosh from the booster until it gets to a few inches from the floor
and the pedal firms up and the brakes operate (stopping power).
So I replaced the master cylinder with a NAPA part and bench bled it for a good 20 strokes until only fluid was recirculating.
Installed MC and then bled brakes again (full quart) until no air came from bleeders.
I still have the same problem.
The pedal is high and firm and stays firm when I stand on it with engine off.
Start the engine and the pedal sinks nearly to the floor before it works.
Thanks, I'll try it with the engine running. The problem is it won't build up a pedal no
matter how much you pump the brakes with the engine running. It just keeps making
wooshing air sound and goes down to about 2" off the floor. I know that sounds like a
master cylinder but it is new and bench bled and it has a good pedal when the engine
isn't running or the booster isn't working. Is it possible the atmospheric check valve is
bad in the booster causing an over boost that is pushing fluid past the master cylinder
seals? Just another wild guess. This one has me stumped.
In most brake systems there is a valve a shuttle valve gizmo that moves to one side or the other when you have a front or rear brake circut failure.. When the brakes are working correctly the spool is centered when you lose one side the spool gets thrown off center..Don't know if you have one on your vehicle or not. Follow the brake lines. Front and rear will have to go to it..After you fix the brakes sometimes a sharp rap on the brake pedal will center it..
If not you have a leak or air..Air compresses more with engine running.
Were you able to loosen the bleeders. Sometimes calipers get air trapped in them..Do the pump it up hold down bleed method? Gets air out best. Check calipers? Pads? Hoses..?
If I had to guess, I'd say there;s still air somewhere. Please don't get offended if this seems too basic, but brake bleeding takes a ton of patience sometimes. First, make very sure all the bleeders are at the top of the calipers. Sometimes it's easy to install the wrong caliper on the wrong side and wind up with the bleeder at the bottom rather than the top. Only fluid will come out because all the air is trapped in the caliper.
Second, as I found out when I restored my '68 Mustang, replacing all the brake lines means everything is dry and all those kinks to get over stuff are great pockets for air to hide. I start with the wheel farthest from the master cylinder (passenger's side rear) and bleed it until you think you have all the air out. Next move to the driver's side rear and work on it, then move back to the other rear. Once you think you have the whole rear done, move on to the front, doing the passenger side front first, then the driver's side front. Then start over, and see if you find air somewhere. Keep an eye on the master cylinder's fluid level and keep looking for leaks. If it's not leaking then it HAS to be air somewhere. If you really can't get it, try one of those bleeders that force fluid up to the master cylinder from the calipers/wheel cylinders.
You just solved your own problemo. Bleed the RABS unit too. I bet you've got trapped air in it. I would bleed down the RABS, then the rr\, then l\r, then r\f, then RABS again, then l\f. Always start farthest from master cylinder and work toward it.
I'm having a very identicle issue on my 05 f250 5.4 any ideas? and how do I bleed the RABS, i did't know that you could, also, if I'm going kinda fast, and then slam my brakes (VA drivers suck) then sometimes my front right side will lock up, until I push on the brakes kinda hard again and then that side will release, almost cause me to drift off the road the other day doing about 60 mph... but I also have stiff pedal when engine is off, and then when engine is on, pedal slowly goes down until just a few inches from the floor... really need to get this fixed..