unit505

How is the vacuum to the booster? At this point, I really think you need to go to Oreilly's or somewhere with a good return policy and purchase a brake booster new and try it. If it is not the problem, I've never had issue with Oreilly's allowing me to return a part with no restocking charge.

MT-Ford

I had the same problem. I went through the entire brake system including two different master cylinder's and still soft pedal. Then I was told by a Ford guru to put on an F-450 Superduty master cylinder on my 90 F-250 and the pedal has never been firmer and stops on a dime.

nfld96ford

I'm going to focus on the booster next (vacuum to it first and if that's good get another booster and try it). My experience from booster failure is usually a rock hard pedal and excessive pedal application effort to effect a stop, though I am aware it may not always be like that.
I can't see how the hoses could cause this if they aren't leaking or collapsed. I actually bought new hoses and couldn't bring myself to install them as they were in such good shape already. There is excellent flow of fluid to the calipers/wheel cylinders so I doubt it's a blockage, leak or significant air in the lines kind of problem.
I'll try those couple of things and see how it goes. Getting a little frustrated with this now...

tway

[QUOTE=nfld96ford;3629808]The firewall, mounting bracket, booster and master cylinder attaching nuts and brake rod are all normal with no apparent damage. The booster+master cylinder moves ahead and back ever so slightly on hard pedal application/release but nothing to cause the pedal to travel the way it does and it looks like the firewall does flex a little but there is no cracks or compromising damage there.

Any other ideas? I'm getting good and poisoned with this.


I had the same problem with my 93 f150. Brake booster was bad. meaning it wasn't assisting the push on the rod of the master cylinder. I think some brake fluid leaked past my master cylinder seals and got into the booster. I changed the booster which fixed the problem then after I also changed my master cylinder.
I did notice with mine that there was a slight hiss if I listened close when truck was running but it wasn't much to hear.
There are three quick test you can do for a brake booster. Two of them don't mean much for your problem I found test 1 was only one I could use really.
Test 1 engine off pump brakes and hold lightly start engine. Brake pedal should drop 1/4 inch or so but not rise. (Mine was rising)
Test 2 run engine then turn off. Press pedal slowly. First pump should have low pedal, second and third should have a firmer pedal.
Test 3 start engine allow to run for a minute. Push and hold pedal with light pressure. Turn engine off. Hold pedal for 30 seconds should not drop.

Buddhaman

I just went through the same thing with my '95. I replaced pads, rotors, shoes, master cylinder, and bled the system numerous times and still no dice. I noticed that when I stopped the pedal was going to the floor and when fully stopped, I'd take my foot off the pedal quickly and push it again and it was rock solid. Just got done changing the booster and I finally have normal brakes again. The JY I went to tested the booster before I bought at and it only cost $35+ tax.

nfld96ford

Mine seems to have gotten better on it's own. Pedal is good now.
I just hope it lasts...lol
Thanks for the suggestions!

Buckoff

Having the same problem on my 91 5.0l xlt. Changed everything but the brake booster and lines. Ordering new brake booster with installed MC tonight. Tagging so I can find this thread again and to bring it to the top in case anyone has useful tips on installing a new brake booster with master cylinder.

Chris_1

iTrader: (1)

Pedal bleed down at a traffic light or other prolonged stop with no leaks in the system is classic master cylinder symptom. Rebuilts are not always rebuilt well.
Booster symptom is loss or partial loss of power assist.

tway

While a master cylinder is a more typical failure it is not uncommon for a booster to fail over time or due to a master cylinder leaking brake fluid into the booster and damaging the diaphragms or other parts inside. A failed booster normally will create a hard pedal. Meaning it takes a lot of foot pressure to get The brake pedal to move. However it can also give the symptoms of the pedal going all the way to the floor. It is all according to what has failed inside the booster. If the vehicle appears to have good pedal until started and then immediately looses the pedal the booster can be at fault. There is a special tool for testing boosters. Anyway the pedal can sink to the floor but with maximum pressure, like your trying to push the pedal through the floor, the vehicle shoud still come to a much slower than normal stop because the brake system is secure (no leaks) and there is still a mechanical connection from the brake pedal to the master cylinder. It won't seem like much braking however. On a previous post on this thread there are some simple tests provided that can be done to aid in troubleshooting a brake booster problem.

Buckoff

Yup. The brake is going to the floor and I feel like I'm going to push the pedal through the floor before the truck stops. I also have the same symptom of pressure building to a hard pedal when the truck is off then immediately dropping to the floor when the key is turned on. I could hear a little hissing sound associated with the brakes but I didn't dig for the sound. It may be the firewall plunger or the booster or the MC but for 70 bucks reman, I'm not going to fuss and complain over which is which or even diagnose it any farther. The booster and MC are getting changed this weekend. I'll share the results.