Soft Brake Pedal

[Diesel Max]

Ok here goes................. 93 F250 4X4 -- ABS in rear only. I repaced my rear wheel axle seals and rear brake shoes this weekend. I also vacuum bled all 4 wheels to flush the system because the fluid was about the color of root beer (this is where I think the problem comes in). After completeing all the work, I started the truck and had the brake and ABS light lite up on the dashboard. When pushing the pedal with steady pressure, it slowly sinks to the floor. If I pump the pedal, the pedal pumps up and gets firm but then applying steady pressure, it again slowly sinks to the floor. The brakes do work on the truck and stop it quite well. It's just the sinking pedal that I cannot get a grasp on. Below is what I have done to this point.


Bled the master cylinder, rabs valve and all 4 wheels numerous times. I have bled eveything with the vehicle off as well as running. (have bled over a quart of brake fluid through the system at this point) I use a clear hose on the bleeder valve and see no air bubbles commng out any where.

Rear brake shoes are adjusted correctly after replacement.

I have vacuum bleed as well as pedal bleed.

I have used master cylinder plugs to plug the two lines (holes) comming out of the master cylinder, to verify it the master is not bad. When master is plugged, pedal holds fine and frim and does not sink.

I have clamped off the rear brake lines, check the pedal with only the front lines normal and the pedal still sinks.

When I leave the rear rubber brake hose unclamped and clamp the left front caliper brake hose, the pedal firms up and sinks very, very slow. When I clamp both the right and left front rubber caliper hoses, the pedal is firm and acts normal.

So............. logically thinking, there must be air in the front calipers right ???? Well, I have bled atleast 1/2 quart of brake fluid through the front calipers and I see no air comming out.

As a side note, after I bled the entire system, the brake light and ABS light are now out.

There are absolutely no leaks of any kind any where and the fluid in the master resivor never drops.


I am at a complete loss here............ Has anyone ever experieced anything like this and can anyone give me any ideas of what to look for or what the problem could be ??? No work was performed on the front brakes with the exception of bleeding them.

Thanks for any help................

 

[franklin2]

Are the lines and the bleeders at the top of the front calipers? I am not sure if they can get mixed up, but I have mixed up the rear wheel cylinders before, and the bleeder ended up on the bottom, and I never was able to get a firm pedal till I figured out my mistake, and got the bleeders on top.

Parking the truck with the front uphill would also help promote the air to leave the front system and go out the bleeders.

I agree, you have done a lot of good testing, and it has to be something with the front calipers.

 

[Diesel Max]

Thanks for the response. Yes, the bleeders are at the top and no changes were done to the front brake system other then bleeding the brakes. Everything was fine before I bled the system and replaced the rear shoes. I'll try parking it up hill and bleeding with the front parked up hill. Thanks

 

[bigrandy]

This happened on my 92 and I ended up not having the rears adjusted tight enough.Set em the way I normally would but the pedal just wouldnt hold,so I tightened em a couple clicks at a time and it fixed the problem.Might not be your problem but it worked for me.Good luck

 

[Agnem]

How did you manage to mount a wheel cylinder upside down? The bolts are not symetrical (at least on the DANA axles). Worse case is your pipe connection is facing the wrong way.

 

[Diesel Max]

Nope........... Rear Brakes are adjusted properly. E-brake hold in about 3 clicks pushed down and drums have a very slight rub on shoes when turned.

Thanks for the input though......... Anyone else with any ideas ???

 

[Agnem]

I've had this problem in the Moosestang since my 8 lug conversion up front. I'm thinking it's air in the lines, but it comes and goes so I dunno.

 

[smolkin]

After replacing everything brake-wise at all four wheels on my truck I still had a sinking pedal. It ended up inexplicably being the master cylinder, which I also had tested with plugs and had appeared tight. After replacing it I can now stop a train with these brakes. I know you tested the master but since they aren't super expensive you might replace/rebuild it. I'm not a brake expert, just my personal experience with my truck, and I can't explain why it fixed the problem.

Also: if you do replace the master, don't forget to check the adjustment on the booster, there's a little screw at the end of the plunger. If you knew that already, sorry. I didn't know before I did my brakes, it was a learning experience.

 

[franklin2]

How did you manage to mount a wheel cylinder upside down? The bolts are not symetrical (at least on the DANA axles). Worse case is your pipe connection is facing the wrong way.

I didn't know if it was possible on these trucks, my mistake was made on one of those stupid GM wheel cylinders that don't even bolt in, they snap in with a retainer clip.

And I am also not sure the front calipers can be swapped from side to side on these trucks, but if it's possible, the bleeder would end up on the bottom. I guess that really would not apply in this situation, since he had good brakes before, and never messed with the front brakes.

 

[Popeye2347]

I have almost the same story with the same problem. In addition to calipers/wheel cylinders I replaced the master cylinder. That did not change the problem. I spoke to the original owner whose comment was 'They always stopped me.' So with that I have been driving it now for several months without problems. Not that I have a 'warm fuzzy' feeling but as he said, they never fail to stop the truck.
I think it is a bad or under performing vacuum pump, not maintaining enough vacuum at idle to maintain a firm pedal. I have not yet tried to check the vacuum but will once warm weather returns. And if that looks good I may get industrious and check the RABS valve. That is the last item I can think of to check.....other than a check at the brake repair shop---not!<grin>

 

[Diesel Max]

Thanks for all the feedback. At least I know it has happened to other people and it's not just me. I have a new master cylinder and will probably put that in tonight, just to eliminate that all together, although I don't have a good feeling about that fixing it.

As stated above the truck does stop but if you are sitting on an incline holding constant pressure on the brakes and the pedal gets to the floor is it still going to hold ????

Could it be that the brakes on these trucks need to be power bled with higher pressure to release all air from the system due to the rear ABS ??

Since the brakes were working 100% before the rear shoe replacement and the flushing/bleeding of the brake system, I would like to think it has nothing to do with the vacuum pump since that was not touched at all during the process but I suppose anything is possible.

 

[Diesel Max]

One other thing to add.............. Pedal does firm up and stays pretty firm with very little movement when the truck in not running. Seems to sink much faster when engine is running.

Seems like when I have had vacuum booster problems in the past on other vehicles the pedal tends to stay hard or firm with no intial movement when first pressed unlike when the vacuum booster is working properly, there is an intial soft feeling to the pedal and then it gets hard or firmer the further you push it down.

 

[Popeye2347]

Uh huh.......let us know what fixes it if/when you discover what is causing this problem. I for one am very interested in the outcome.

 

[Diesel Max]

Will do.............. Hopefully a few more folks will chime in with some more ideas or experiences...........

 

[smolkin]

Just gonna add that the vacuum only helps you apply the brakes, I think, and has no bearing on their performance. If you've ever lost vacuum while driving, you know what I mean. Happened to me, and although I had to STAND on the pedal (I only weigh about 155) it does stop ok without vacuum. It was the C6 not shifting that really sucked.