Alright, just spent a while reading up old threads about soft brakes and now have a few questions.

Background: 95 Bronco 351, I recently began driving it and am slowly getting around to its little problems. It has a soft brake pedal, lots of pedal travel until the brakes engage but it then stops fine. I replaced the master cylinder (i did bench bleed it) and then bleed all the lines. I did this until there was fresh fluid at each caliper and started with the furthest away working my way in.

After completing this the brakes felt nearly identical as they did before I began. I can't hear any leaks from the booster and hear no hissing when the brakes are applied. I also did not visually see any fluid leaks, nor had the fluid level changed.

So... there it is. Reading previous posts two items that caught my eye were vacuum leaks (maybe i didn't hear it, but still.. would this cause a hard of soft pedal?) and then the pushrod adjustment for the master cylinder. What are the guidelines for the pushrod, and how do you know if this could be the issue? One better, what would cause it to get out of adjustment?

Thanks for any guidance!

EDIT: The more I think about it, when I put the new master cylinder on should I have felt it hitting the pushrod? Should there be some pressure applied? It went straight against the firewall seemingly without any real contact with the pushrod.

 

Not sure about the push-rod adjustment issue but then I've never had to deal with this particular issue myself. I can't see how any adjustment there would cause a soft pedal though since as I know it, it is a purely mechanical link.

The brake booster utilizes a vacuum check valve so that as manifold pressure changes the amount of vacuum available to the booster does not fluctuate. While I would consider such a failure exceedingly rare, its not beyond the realm of possibility. The check valve is the 90 degree fitting in the booster that connects the booster to the vacuum supply.

 

brake booster faliure or problems would cause a hard
brake pedal, the booster only provides assist in
applying pressure to the master cylinder.
If your not leaking fluid, your drum brake self-
adjusters might not be working, that is where I
would start. Make sure everything moves freely
and the star wheels spin smoothly, and adjust
the shoes out until the drum is snug aginst them.
Also make sure the front calipers
move freely on thier slides, if your front pads
are wearing unevenly that is a sign your caliper
slides are binding and have to travel much farther
to engage the brakes. A soft pedal is almost
always brakes out of adjustment or air in the system.
You eliminated the other causes with the new Master
cylinder.

Good luck
Ed

 

Um, brake booster check valve problems can indeed cause soft pedal issues because if the vacuum supply to the booster is not regulated by said check valve the excess vacuum will give you more assist than necessary and cause the pedal to feel soft. Not all booster problems are caused by a loss of vacuum. Booster failure itself... yes, hard pedal because there is no vacuum assist. Check valve failure is a different animal.

 

That's not true, a booster can never cause a soft or low pedal. The check valve's purpose is to retain vacuum in the booster in the event that the engine quits and you need the vacuum assist to stop the truck one last time. It doesn't do any regulating of the vacuum at all. It's a simple, one-way check valve.

I have a bad check valve in my Bronco, I just discovered it and I haven't gotten around to fixing it yet. I know it's bad because I was climbing a long steep hill at near WOT for 4-5 minutes and when I got to the top I stopped found I had no power brakes until I let the vacuum build back up, which only took a second. But it was a scary second because you don't want to be on top of a mountain with no power brakes!

 

I found out a neat trick for deturmining if you have a vacume leak. Just turn the AC on max and floor the throttle. If the AC stops blowing (the door shuts) you have a leak. It doesnt take a big leak to do this. Of course, it doesnt help you find it.

 

Soft brake hoses can cause a soft pedal as tyhey will swell before pressure can build in the caliper. I had the same problem on an 87 3/4 ford I had years ago.

 

Andym said it perfectly, If you run the truck then shut it off
and pump the brake pedal and get at least 2 assisted
brake applications before the pedal gets hard
your booster and check valve are OK.

Ed

 

So I tried the pedal after turning her off and I do have the 2 asssted brake pushes so I guess that rules out the booster and check valve - thanks yall.

Looks like i've narrowed it down to either the brake hoses, there still being air in the lines (although i completely flushed with new fluid, gravity bled, and bled with pedal application until there were zero bubbles), or the fact that the master cylinder I bought earlier was the 1 1/16" bore and not the 1 1/8" bore that ford began using later. Could that extra 1/16" make this big of a difference? If the 1/16" bore cylinder was originally used wouldn't that mean that they had soft brakes from the factory? weird.

Thanks for all the help.

 

you also have 4 wheel abs and i think you have to bleed the brakes with the engine running if i recall. are you following the haynes directions?

 

The pushrod adjustment isnt really anything more than making sure the push rod lets the master cylinder completely relax, but doesnt loose contact with the master cylinder piston. You can adjust it out to cause the brakes to react sooner but that is only masking the real problem of why the brakes are soft to start and eventually you will have the same problem as the fluid pressure bleeds back into the master between applying the brakes.

Kem mentioned your 4wheel ABS, that system bleeds may differently than the old RABS and non-ABS system, and to top it off every manufacturer has a different bleed procedure, if not every vehicle. Some areas to look at are soft metal lines, and spoungey rubber lines.

I kind of like the idea of the braided stainless lines, but my budget doesnt include those at the moment. Also look at rotors or drums that are not within the recommended thickness spec. The drum brake tends to go back to the same rest spot on these trucks unless your adjuster is just under the slave cylinder (like it was on a 91 t-bird I had). The further this has to travel to meet the drum, the more pedal travel you have to go through before the brake begins to work on the truck's rear axle.