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Have 1990 Ford F250 with 460. Complaint of hiss noise with brake application and pedal will go to the floor. Bled brakes. Had good solid pedal and no fade with engine off. Start engine and pedal is soft and will go to floor. Will not pump up. Obviously bad booster and master cylinder.

Replaced with loaded booster. Bled all brakes. Still same issue. All brakes are adjusted properly. Turn off engine and pedal is good and solid with no fade.

Seems like I remember a problem on those trucks having to small of a bore in the master cylinder and causing the caliper brackets to flex when appling the brakes with vacuum assist. If you clamp off the brake hoses, does the pedal get hard? If so, have someone release the line clamps one at a time and see which one causes the pedal drop and folow it from there.

Do not, I repeat Do Not condem a master cylinder or any other brake issue on these vehicles while sitting still. How does it act while driving? Pedal going almost to the floor while stopped is normal in my past experience with those 460's. Push through is what a Ford DSE told me.

Mitchell1 On Demand has a TSB # 91-20-8 from Ford for more information on this problem.The issue is with the front calipers. I did in a few cases install a master cylinder with a larger diameter bore to help in this fading issue. So from past experience I have to disagree with a previous post.

agree with ric. pedal will go to floor at a stand still. how does it brake when driving? that's the real test. there are way's to better the pedal but and i mean but, don't star throwing parts at it because all may be as good as it will get. the action while driving is the only true way to deal with this.

does this beast have one of those check valves?

if you're talking about a rabs valve i don't think so. this may be a result of improper e/brake adjustment causing a drag that will make you think that it's an adjustment drag. there were fords that i moved the e/brake anchor to release pressure and allowing a better adjustment. if this is 4 w disc i've seen issues where the slides are frozen. wheel bearings will cause issues as well. i believe ric hit this one. many ford trucks will appear to have fading brake pedals. if the person just purchased this truck and is comparing the pedal to other trucks it will seem to fade away. if you try you can push the pedal to the floor at a stop. again, if you throw boosters and masters at it you'll be wasting your time and your customers money. on the other hand if it has a rabs valve like tina mentioned, that can be a problem. there is a bleed screw on them and they will hold air and need to be bled to attain a pedal.

I agree with the RABS valve issue but I have also seen some with the caliper issues described in the TSB from FOMOCO I listed in an earlier post. However, make sure the integrity of the rest of the brake system is good before you proceed with the TSB repairs. Rear brakes out of adjustment can also produce similar sinking pedal on them.

nah, i was talking about the plastic check valve on the booster motormite 80190 and 80191 for the grommet asst.

Had another vehicle this last week from this customer and asked about the F250. Said he hadnt driven it much as it is a secondary vehicle but seems to be ok with it. It does stop well even with a low pedal.

We will call this one fixed!

i just love closed cases where the customer understands the end results.

It is always good to have an understanding customer. Wish that they all could be that way.

amen to that..............