Soft Brake Pedal

[THECACKLER]

Well from my experience, a sinking pedal is a fluid leak or bad master cyl. A hard pedal is a power booster issue, whether low or leaking vacuum or booster check valve or diaphragm. A spongy soft pedal is air or soft hoses. There's a pin on the proportioning valve that usually has to be pulled or pushed to bleed the system properly. Generally a two man operation. Check your service manual for the right way or perhaps someone will chime in. FYI .. My pedal always went too close to the floor for comfort until I hydro-boosted it.

 

[BlueBreadTruck]

I am also interested. I just did a rear brake job on a 94' f-250 and replaced the wheel cyls. bled the hell out of it, and no bubbles at all, STILL has a soft pedal....before it was tight! the brake shoe adjustors are out till it slightly drags also!

 

[franklin2]

For an experiment, I would put the plastic plugs in the master cylinder, take it off and put it in a vise, and push on the plunger with something and see what it feels like. If it sinks, then that is what you were seeing in the truck. If it moves a little bit and then stops, it has some air in the master. The plunger shouldn't move at all when it's fully bench bled.

 

[Diesel Max]

Ya, I think I'm going to mess with the master cylinder tonight just to try and rule that out again. I know when I plugged the master the pedal did not move. Also when I clamped the rubber hoses that go from the frame to the caliper on both the left and the right calipers, that too stopped the sinking pedal, so that doesn't seem like it would be a bad master cylinder. Seems like it would be air in the front calipers but I bled 3/4 of a quart of brake fluid through the 2 front calipers and the pedal still sinks.........


If it turns out Not to be the master cylinder, then I'm going to borrow my buddies power bleeder and power bleed the system, don't know what else to do at this point.......

 

[sassyrel]

Ya, I think I'm going to mess with the master cylinder tonight just to try and rule that out again. I know when I plugged the master the pedal did not move. Also when I clamped the rubber hoses that go from the frame to the caliper on both the left and the right calipers, that too stopped the sinking pedal, so that doesn't seem like it would be a bad master cylinder. Seems like it would be air in the front calipers but I bled 3/4 of a quart of brake fluid through the 2 front calipers and the pedal still sinks.........


If it turns out Not to be the master cylinder, then I'm going to borrow my buddies power bleeder and power bleed the system, don't know what else to do at this point.......

annddddddddd---just because you get a rebuilt mc--doesnt mean its good!!!!!!!!!!! been there--one too may shirts--DO THE BENCH BLEED, till firm--some ive had to fight awhile--others--easy peasy--

 

[J D Recovery]

lost my vac pump the day I got my truck. Was going from norfolk VA to above balt MD. Truck stoped fine just had to use two feet to push the pedal. My .02

 

[GOOSE]

to check your vaccuum tee tap a gauge into your system. With truck running, you should achieve 20+ inches of vaccuum. Turn the truck off and it should have less then 3 inches of vaccuum leakdown in one minute. This is as per the factory manual. I was troubleshooting same system one day. Replaced master cyllinder, problem didn't go away. Found "coffee can" reserveoir was leaking. Test booster. I believe with vaccuum generated and truck off, press pedal as far as can to floor and release one time. If vaccuum goes to zero, booster is bad. Bad part #3 that day. Replaced those parts and bled the snot out of it, improvement but still had sinking pedal. The guy I was helping ran out of money and ran with it for the time being. Reading this thread, I should call him and see if he is ready to continue. I can't logically see how it would create this problem, but thr RABS has me scratching my head. I always wanted to ditch that pos and plumb a wilwood adjustable proportioning valve into the cab. Someday. Anyhow, I was able to save my buddy from buying a vaccuum pump, at least. Also, a new MC was about $7 more and had the reservoir included, money well spent as far as I'm concerned. Hope these tips can eliminate some guess work for you.

 

[93cc7.3]

i to have this same issue my abs light is on and my brakes works great but sink if u press hard

 

[Diesel Max]

Replaced the master cylinder last night. Brakes are better and stop the truck great but still have the sinking pedal but it doesn't go all the way to the floor anymore (maybe 3/4 of the way to the floor). Bench bled the master cylinder before installation and bled the ^&*(* out of the the wheels and the RABS valve.

I still am of the opinion 1 of 2 things are going on here. Either I still have some air in the system and it needs to be power bled OR the RABS valve is dirty or not functioning correclty. I have read a dirty or broken RABS valve can cause a sinking pedal as well.

With any luck, I'll power bleed the system this weekend and will post an update here on the forum at that point.

 

[franklin2]

I know this is getting to be a bad habit of mine, but if it's broke or reliability is getting to be a problem , and it's usefulness is in question anyway, I usually get rid of it. I am talking about the RABS valve. I ran a new line from the master cylinder to the rear line and coupled them together, bypassing the RABS. So no more pedal problems caused by the RABS.

 

[Diesel Max]

I know this is getting to be a bad habit of mine, but if it's broke or reliability is getting to be a problem , and it's usefulness is in question anyway, I usually get rid of it. I am talking about the RABS valve. I ran a new line from the master cylinder to the rear line and coupled them together, bypassing the RABS. So no more pedal problems caused by the RABS.

After you removed the RABS and ran the brake lines directly to the rear axle, how does everything work ?? Any Brake lag or any problems ??

 

[franklin2]

After you removed the RABS and ran the brake lines directly to the rear axle, how does everything work ?? Any Brake lag or any problems ??

Everything is normal. My braking system works like any other older truck that didn't have the RABS on it. The only hard part of the bypass is cutting the universal piece of line you get at the store, and re-flaring it with the oddball sized nut the factory used to go into the master cylinder. The ports on the RABS are standard sized, so the standard sized 3/16 nut on the universal lines and a standard 3/16 coupler works down there.

 

[catodd]

I was having the same problem, brake booster was loosing vacuum, replaced brake booster.....no more problems.

 

[Diesel Max]

I was having the same problem, brake booster was loosing vacuum, replaced brake booster.....no more problems.

Interesting...... My experience with bad vacuum or brake boosters in the past have always been a hard firm pedal that does not stop the vehicle unless you use both feet. In my case the brakes work very well but the pedal sinks when continous firm pressure is applied.

I wouldn't think the booster would be the issue as it appears to be working fine but the problem is still a mystery at this point so I guess nothing can be ruled out.

 

[Jason R]

I just replaced the front callipers on my '94 f250, and now have the same issues as you described. I know this problem is a very common occurrence, every Ford related forum out there has this very same problem described, but so far no one has a "complete" fix. My brakes and pedal height were fine before I replaced the calipers, and now with engine running the pedal goes almost to the floor then slowly, slowly sinks. I tried cleaning out the RABs valve by tearing it all apart, that didn't solve it. Replaced the Rabs valve. That didn't do it. I haven't adjusted the rear brakes yet, but by the sounds of it, that doesn't help, either.

It's very puzzling that almost everyone had a properly working brake system before doing simple maintenance, and now are suddenly struck with this problem.