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Old Jul 7, 2007 | 09:09 AM
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idifamilycab
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brakes

wondering why my my brake pedal slowly goes to the floor. seems as though there is a leak. looked at my rear wheel cylinders and they may leak just a little but its not obvious.ie i dont see brake fluid on the insides of my wheels or leaking off the drum. I do need to replace my rear axle seals though because my brakes do have a trace of gear fluid on them. I did just replace my front rotors, pads , repacked wheel bearings and bled the whole systrm 5 times just to try and get any air out of the system. Still after all that the pedal slowly goes to the floor. brake fluid also stays at the same level in the reservoir. could my master cylinder be bad? oh well any help would be appreciated. yes i do have vaccuum going to my booster.
 
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Old Jul 7, 2007 | 10:22 AM
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i had "that sinking feeling" in my brakes for 19 years.
a few months ago, i modified a F450 master cylinder to fit. no more pedal to the floor at a taffic light anymore.

the stock 250-350 master uses a 1 1/8 piston
the 350 master uses a 1 5/8 piston.
 
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Old Jul 7, 2007 | 01:42 PM
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Had the same problem on my `87 F250 a couple of years ago. Turned out to be a faulty rear anti-lock brake valve.
 
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Old Jul 7, 2007 | 08:11 PM
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Ive had that problem it turned out to be fluid bleeding past the o rings in the master cylinder you wont necessarily lose fluid when this happens.
 
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Old Jul 8, 2007 | 07:46 AM
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so what should i do? I ve thought about putting the 450 master cylinder on and of course elongating the mounting holes to make it fit. they cost about the same amount of money. also do the brake lines mate up to the 450 master or do i need to adapt them? my abs does work though.
 
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Old Jul 8, 2007 | 08:49 AM
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the lines bolt rite up. you need to elongate the mounting holes to make the 450 master fit, and you need to clean the inside of the booster. it is such a tight fit, that i had to pull it into the booster with the nuts. the booster cup is the same size as the master on the 450 unit, the stock 250-350 master is 1/8 inch smaller than the booster cup.

the added advantage of using the 450 master is a 1/2 inch larger piston in the master, and a larger fluid reservoir.
the larger piston moves more fluid, so you do not need as much pedal pressure to stop.
i had to relearn how to drive my 88 after doing the 450 master swap. before, it was all you could do to stop.
now, if i look at the brake pedal too hard, i face plant into the windshield.
 
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Old Jul 8, 2007 | 09:39 AM
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well thanks for the info i probably will do that as soon as i get up enough cash. i will let you know how i goes
 
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Old Jul 8, 2007 | 09:42 AM
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well i got the same problem on my 94 i have replaced the master cylinder the wheel cylinders and new brake line through out the truck new calibers new pads and i still have the sinking to the floor as soon as i figure it out i will post back,
 
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Old Jul 9, 2007 | 05:06 PM
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Originally Posted by ncfirefighter46
well i got the same problem on my 94 i have replaced the master cylinder the wheel cylinders and new brake line through out the truck new calibers new pads and i still have the sinking to the floor as soon as i figure it out i will post back,
well, find out as quickly as you can so i can do the same to mine, only i get to cheat and save money. any ways i went to autozone to price the 450 master and they wanted $80 something bucks for a reman w/out a reservoir, $141 or so for a new master w/reservoir, or about $39 for a reman 350 unit. looks like ive figured out which one im gettin. that stinks...
 
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Old Jul 9, 2007 | 06:26 PM
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Originally Posted by tjc transport

the added advantage of using the 450 master is a 1/2 inch larger piston in the master, and a larger fluid reservoir.
the larger piston moves more fluid, so you do not need as much pedal pressure to stop.
i had to relearn how to drive my 88 after doing the 450 master swap. before, it was all you could do to stop.
now, if i look at the brake pedal too hard, i face plant into the windshield.
I am going to have to disagree here. I am not doubting your experience of better braking with the swap, but to achieve the same psi braking, the larger piston in the 450 master cylinder will require more pedal pressure, but less pedal travel, if the wheel cylinders and caliper pistons stay the same diameter. A smaller piston in the master cylinder will require less pedal pressure, but more pedal travel.

That's how a hydraulic jack works. Many strokes of the little cylinder lift the large cylinder. The ratio of the diameters of the piston and the jack cylinder are what you give you the advantage to lift lots of weight.
 
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Old Jul 9, 2007 | 07:04 PM
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i agree with you dave, i stated it wrong. you do not need to push it almost to the floor to stop. now you only have to touch it. after having no brakes for 19 years, it is such a pleasure to drive it now, i am almost tempted to park the crown vic and make the diesel the daily driver again.
 
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