Hey guys, kinda new to posting here but could use some help. I have a 2002 F150, great truck, never had a problem. About a month ago, went to stop and brake pedal almost went to floor. It stopped but it was not a good feeling. Pulled over, checked MC fluid, any leakage at all four calipers, everything seemed fine. Drove some more and found myself having to pump brake pedal to firm up brakes and stop........Assumed leaking master cylinder and replaced with new, not rebuilt, from Autozone. Drove truck and seems to stop maybe a little better, but nowhere like before- very spongy pedal. I bench bled master cylinder before replacing until it was almost impossible to push plunger in by hand. Bled brake lines at all four wheels probably 15 times per wheel with a helper pumping the brakes.
Brake pedal is rock solid after pushing maybe twice with engine off, but if you hold medium pressure on pedal and start engine, pedal probably sinks 3 inches to firm up but still feels spongy.Anyone have any ideas or how to check brake booster?? Does ABS have any bearing on this???? Any help would be appreciated!! Shouldn't have to pump brake pedal to firm up brakes.

 

What you describe is a booster that passed the booster test.

Vacuum Booster Test:

1) With engine running, foot NOT on brake, turn engine off.

2) Press down brake pedal with moderate pressure, and release. Do this again, and then again. A typical OK booster will have 2 to 2 1/2 boosted brake applications, until the vacuum reserve in the booster is depleted. When vac reserve is depleted, the pedal will become very hard, and will require a lot of force to depress pedal to get much pedal travel... that is what un-boosted brakes feel like.

3) With foot on pedal with moderate pressure, start engine. The pedal should drop, because vacuum boost comes back, to multiply your effort.

If it does all that, the booster is fine.

For most ABS systems, if you get any air into the ABS unit, then the proper communication tool is needed to open/close valves in the right order to bleed the unit. Luckily, I have never gotten air into the ABS unit of any of the vehicles I have had with ABS.

But people often miss things on brake systems.
Like a brake hose that is externally cracked and the reinforcing braid is breaking, allowing the hose to swell slightly during brake application. Just a small rubbery increase in pressurized brake system volume will require a lot more stroke of the master cylinder.

Fluid leaks - can happen anywhere, including tiny leaks in steel lines due to rust-through.

I assume you have disks at all 4. Any caliper hanging up and getting cocked to a side?

A good thorough visual can often save a lot of part replacing.

 

Thanks for the advice, I'll do a very thorough visual check as soon as weather permits(raining now and suppose to for a day or so) Any way to isolate just the booster and master cylinder, and then booster, master cylinder, and ABS valve to find what section the problem might be occurring in??

 

Not that I can think of. The ABS can isolate individual hydraulic circuits from the master, but there is no (easy) way to get a pressure gauge in there to see. There might be some diagnosis that could be done with the proper scan/comm tool that can talk to the ABS controller, I have not looked in the manual for it. It would require the tool, probably big $$.

 

Well there is definetly a bleed procedure that involves the ABS's Hydrolic Control Unit, or HCU for short. Once air is introduced into it, it can be hell to get out. Ford hooks up their diagnostic equipment to it. Then via the ABS module it activates the HCU motor and the inlet and outlet valves and works the air out.

Another thing i have seen before. Though a rare occasion i have seen it cause your symptom. I've seen a really loose wheel bearing cause that problem too. You pump up the brakes and make them solid but once you let off the pedal the weight of the vehicle causes the wheel bearing in question to settle again and when the wheel tilts in(or out) it uses the rotor to push in on the caliper pistons and then the next time you hit the brakes you have to pump up the brakes again. I have seen it happen.