I have a 1998 F-150, 4.6, 2WD, Rear ABS. I am completely rebuilding the brake system. I have replaced the brake booster, master cylinder, both front calipers, rear wheel cylinders, and proportioning valve.

I have bench-bled the master cylinder with the fancy bleeder syringe, I have bled the brakes every way you can think of...vacuum, gravity, pumping the pedal with engine off and engine running.

I have adjusted the booster pushrod 6 ways to Sunday and still have a mushy, sinking pedal.

When bleeding the rear cylinders, I can only get any measurable amount of fluid to the rear wheels if I use a vacuum pump and disconnect the electrical connector from the aluminum-block-looking-thing under the master cylinder that routes fluid to the rear of the truck. (I obviously don't know the name of this part lol)

If I simply try the old engine-off, open the bleeder screw, have someone pump the pedal method, I get almost no fluid through the bleeder screws on either rear cylinder. The font brakes bleed fine! Lot's of fluid through the bleeder screw on the calipers with every push of the pedal.

I have bled brakes successfully many times but never a vehicle with ABS. Since the front cylinders seem to work great, I'm assuming it has something to do with the ABS system.

The only thing that I know of that I haven't touched is the ABS HCU. But, one thing I did that may have caused a problem there was to inject low-pressure compressed air into the front and rear brake lines where they connect to the master cylinder to push all the old fluid out of the system.

I have NOT bled the HCU but I read somewhere that you can do that by taking a ride on a dirt road and sliding the tires a few times.

What am I missing or doing wrong? Thank you for your help!

 

Don't know much about bleeding the newer brake systems, but one thing that helps with bleeding generally, ensure the rear brake shoes are adjusted all the way out and tight against the drums before bleeding the rear system. Too, if the rear shoes are not adjusted properly the pedal will be low and mushy.

 

Thanks Tedster, I'll check that next.

 

..keep us posted.......There is a lot of people that have issues with the HCU going bad, but usually thats with 4 wheel abs...not sure about rear abs only

 

Ok. I tightened up the rear brake shoes. The RR was pretty tight. The LR had some slack in it. They are both now barely scrubbing the drums when you turn the rear wheels.

It helped a little with the initial pedal pressure but if you pump up the pedal and hold pressure on it, it slowly fades all the way to the floor.

I didn't try bleeding the rear cylinders again. I will do that tomorrow.

 

When you say vacuum bleeder do you mean something like a Motive power bleeder? Pump up the pressure in the bottle that is connected to a cap and it pushs fluid through the system.

Thats what i use on all my vehicles and i havent had any issues like you are talking about.

 

If you're having trouble getting fluid to the rear, check for a leak/loose fitting going to the rear lines.

 

Did you replace the hoses to both front calipers? They could be deteriorating and swelling under pressure.

 

OK, just so everyone is clear on this technique prior to bleeding run the adjusters up just as tight as they will go, the wheel definitely won't turn. in other words the wheel cylinders will be maximally expanded. Then after bleeding back off the adjusters.

 

Thanks for the replies! Let me answer the questions...

auskip07
Yes, it is a hand-held vacuum pump from the local auto parts store.

Icefan71
No obvious leaks or fluid loss.

cb_13
I have not replaced any hoses. But the front calipers bleed fine. The rear cylinders are the problem.

Tedster9
Ok. I'll run the adjusters all the way out and try bleeding the rear again.

 

cb_13
I realized what you meant after I posted the last reply. I will watch the front hoses when I bleed the rear cylinders. Thanks!

 

Ok. I ran the adjusters on the rear brakes out until the drums would not turn at all.

I put new teflon tape on the threads of the bleeder screws to get an air-tight seal on those and used the vacuum pump on the rear cylinders again. Pulled about 4 oz of fluid through the bleeder screw on each rear wheel with NO bubbles.

I put a clear tube on the bleeder screw of the RR wheel cylinder and connected one of those one-man bleeder bottles. I was able to push a couple of ounces of fluid through the bleeder screw by pumping the brake pedal. I did the same thing, with the same results on the LR cylinder.

With all of the bleeder screws closed and the master cylinder full of fluid and the cap in place, I get better pressure initially when I press the pedal but it STILL fades to the floor!

 

Oh! And I put a video camera on the front brake lines and recorded them while I pressed the brake pedal. (All bleeder screws closed) They did not swell or balloon at all the whole time the pedal slowly faded to the floor.

 

Well guys, it turns out it was a bad or damaged new master cylinder that was the problem. My original problem with my brakes was the power booster. I replaced the master cylinder just because it was inexpensive. My old master cylinder was still working. So, today, I put my old master cylinder back on the truck and now I have pretty much a full pedal of brakes! I will do one final bleed of all cylinders tomorrow and I think they will be fine.

Thanks very much for all of your help!

 

Glad the situation got resolved