I have a good brake pedal until I start the truck then it goes to the floor

GHAS

MEMBER
1993 FORD F-350
V8
2WD
MANUAL
214,513 MILES

Hi, Im working on a ford f350 dually, I have a good brake pedal untill I start the truck then it goes to the floor, I have tried 5 master cylinders, 2 have been from ford, two brake boosters, 1from ford, not rebuilt, new hoses, new calipers, new ford pads, new vacume pump, 25 points of vacumme, new rabs valve new rotors and drums, new rear wheel cylinders, rear brake hose, I have searched all over for leaks, no leaks, the problem is in the front brakes, I blocked the rears completly off, I have bleed about 4 gallons of brake fluid threw the system, nothing seems to help this truck, I am a mechanic, been to alot of different schools, but this one is beyond me, Ive also pinched the brake hoses to the front and the pedal feels fine, thought it was calipers so have put two diffrent sets on besides orginal, have plugged rear brakes completly off, acts like master cylinder but out of 5 and 2 new ford oem I cant see that. But I guese anything is possible, any advice would be appreciated. Thanks Gary

3 Replies

MHPAUTOS

MECHANIC
31,937 POSTS

Hi there,

If you are sure that the problem is the front brakes, to prove get 3 brake hose clamps, lock off all flex hoses. Is the pedal high and hard. Take of rear is the pedal still ok. Take of L/H is the pedal ok If no, the problem is in the caliper, look where the bleeder is, Is it low or high on the caliper, if low, try bleeding by backing off the flex hose at the caliper, there may be air trapped high in the caliper, check both sides.

Mark (mhpautos)

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Wednesday, March 17th, 2021 AT 12:26 PM

GHAS

MEMBER
3 POSTS

HI, Im answering my own question, but maybe it will help someone else, the problem I found was that they were sending me the wrong master cylinder, ford only shows two for that truck but they are wrong, the way I found out was went to a junkyard found a truck like mine, pulled the booster and master cylinder, put on truck, had perfect brakes, checked the # on mastercylinder and they were totaly diffrent, so If you change a master cylinder on this year model truck, get the # off the end of it before you exchange it, because youll probably get the wrong one and the brakes will fade, they look the same but are diffrent inside. Gary

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Wednesday, March 17th, 2021 AT 12:26 PM

MHPAUTOS

MECHANIC
31,937 POSTS

Hi Gary,

Thank you for that feed back and well done on spotting that No. I am sure that this information will help someone else at some stage.

Mark (mhpautos)

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Wednesday, March 17th, 2021 AT 12:26 PM