V8orbust

Hi all, I have a 1999 5.4L Lariat. I've always had a pretty soft brake pedal, but recently it got really bad. I took it to Pep Boys and they replaced the rear wheel cylinders and bled the crap out of the lines.

Here's where it gets interesting. They got me a brake pedal, and I had no complaints at first. After about a week though, it went back to soft. Now, when I get in, it's soft, but as I drive it hardens up and my brakes start dragging about 1/4 of the way. I took it back and they went over the whole system again and found nothing wrong. They think it's a vacuum leak, but I/them/my mechanic uncle are all running out of ideas.

Here's a list of everything that we've done. Some was prior to me taking it to Pep Boys.

1. Replaced master cylinder
2. Replaced brake booster check valve.
3. Replaced brake booster vacuum hose clamp
4. Replaced blown rear wheel cylinder and the other side
5. Bled, bled, bled, bled, bled.
6. Replaced rear differential ABS sensor
7. Replaced front brake pads and inspected rear drums.

I'm running out of ideas here. Is there anything that you guys can think of?

serranot

What about the hoses? If there's any room for things to create a soft pedal, it's the rubber brake hoses.

boggin_bronco95

What about the booster itself? Maybe there is a slow leak inside the Diaphragm?

redseal97

here are a few quick tips for checking the flex lines, pump the pedal then get out and open one front bleeder, if any pressure is present besides normal gravity flow that is most likely the side with a bad flex line, you have to pump the pedal everytime you check a different bleeder. also i notice you already replaced your mastercylinder, if it wasn't bench bled properly its a real sob to get the air out of it, best way is a pressure pot for bleeding, also i know on the older gms you had to "Re Home" the motors in the abs unit to purge air if the pedal wouldn't firm up if the system was opened, and one last thing on the brakes sticking, if the mastercylinder is Overfull on fluid IT will cause your brakes to drag. hope this helps

V8orbust

I checked the hoses for ballooning and I didn't see anything. Admittedly, I didn't exactly have the best help on that so I'll check it again later when it stops raining.

As for the brake booster, possibly. If the hardening pedal is a symptom then that (unfortunately) bumps it up the list. I haven't heard any noises and it pulls vacuum. I pulled off the check valve and it went WHOOSH.

I'll have to go out and borrow some of Autozone's tools later. I recently moved out on my own, so my tool situation is lacking. As far as I know, I got all the air out. Then again, same help as checking hoses, so anything is possible. I'll pull it off and redo the process probably tomorrow. I have also heard about air being trapped in the ABS motor. I asked Pep Boys if they activated the ABS while bleeding and they said yes, but I made a mistake and didn't physically watch them.

redseal97

sometimes the hoses won't bulge, they can deteriorate on the inside and that can lead to your calipers sticking on....this won't in itself make a soft pedal but it gives you another avenue to check for the sticking brakes situation, as for the air, the best advice i can give is find a garage with a positive pressure bleeding set up, it may be worth an hours labour charge to be guaranteed to have all the air out, i'll look at my factory repair cds and see what they list as factory bleed procedure.

redseal97

just checked the repair manual, has no mention of air being trapped in abs unit so you should be good there, just standard bleeding practice, rr,lr,rf,lf, they did mention the pressure bleed method as well.

V8orbust

Tell me what you think of this. I think I've found a way to recreate it on a whim, and maybe a solution. When I park on a hill with the bed downhill, the pedal is firm when I get in, and the sticking brakes start soon after. Parking the other way doesn't cause this.

Could brake fluid be leaking past the diaphragm in the booster? Seems like that would cause the hard pedal and the sticking brakes. I have a feeling that if that's the case, it's about time to retire it. She's got 304,000 miles.

redseal97

u can check if brake fluid is leaking into the booster by one of two ways, pull booster hose and look inside, if its wet with fluid replace booster, second check inside at where pedal shaft enters the booster, if its wet with fluid on that seal then also you will have to replace booster, sticky brakes i still say best bet is hoses