i dont know every thing about brakes, but by what ive gathered sounds like the distribution valve is bypassing fluid.

Please enlighten me to what you mean by distribution valve? I'm not sure what you are getting at.

i think on my truck (85) its called a distribution valve , on yours (91) i think they call it a rabs valve.

I've heard it called the metering valve and also the RABS valve. Either case, that's ruled out after today's update. The rear brakes were hooked back up after the M/C passed the with having both ports blocked.

I had no sinking of the pedal with the front brakes disconnected. That valve controls the rear braking. Once I hooked up the front line to the M/C and blocked the R/F caliper from getting an fluid, that's when the pedal started to sink again.

Once again, I only have the sinking pedal with the engine running. There are no signs of fluid loss whatsoever.:-wink

sounds like your going through the same thing to a tee that a friend of mine is having with his truck. new cal,pads,booster,mc(3 to be exact), and still the sinking pedal. im kind of a hands on kind of person, im checking it out fryday so if i figure out what is causing his trouble ill send it your way. i know on my truck i was having the same kind of prob with mine i changed the master cylinder thinking it was bleeding off, but turned out to be the back brakes were out of adjustment. i replaced the shoes and adjusted them problem solved.

I would look into the RABS valve, heres a link that may give you some more info in it
http://www.asashop.org/autoinc/dec99/techtips.htm

[QUOTE=dieself250]I would look into the RABS valve, heres a link that may give you some more info in it
http://www.asashop.org/autoinc/dec99/techtips.htm[/QUOTE]

That link was in my original post. I was suspecting that but not now. That has no control over the front brakes. The rear brakes are solid while the engine is running or not, with the front brakes disconnected. Once I connect one front brake to the M/C the pedal starts to sink with the engine running.

I know the valve you are talking about now. Just for heck of it, try bypassing it for testing purposes. I have heard people say that they can give problems if its like the one on a 95 model.

It's worth a shot.

hey man ive encountered this with a ford tempo or was it taraus.what i did was remove the master cylinder cap and open the bleed screws and let it gravity bleed for 3-5 min close bleeders and check master make sure its not OVER FULL and place cap on and pump up should be good .

check right below the master cylinder on the inside frame rail you should have an ABS valve there is a bleeder there if that does not fix the problem it might be the ABS valve its self. the fluid will bypass in that valve giving you the same symptons as a failing MC. You can bypass the ABS valve all together that is what I did or you can replace it. I think I have seen them at rockauto.com for around 125.00. you can also rebuild it you self be carful not to brake the thick gasket you cannot buy that. I think the valve has 4 or 6 O rings that would need to be replaced.

I forgot to mention if you by pass the valve leave the valve in place with wires conected otherwise you will get a warning light staying on. you can get brass fitting for the bypass.

i've read everythingnow try this -block the rear brake port on the master, see if the front brakes hold if they do its probably the rabs valve

Hay DSSinc-Dont fell bad -I just spent the whole day in the garage and my truck is doing the exact same thing-I have all kinds of brakes till I start the truck-Then it goes right to the floor.I am thinking about starting a fire under it.

Problem Solved!

I've just been really busy for the past month and unable to post the solution for my problem.

PLC7.3 posted a TSB regarding the vacuum booster. I researched the TSB further and this is the complete TSB that I found;

Subject: Low Brake Pedal

Vehicle Involved: 1997 and Earlier F350 Diesel Power Truck with Vacuum Brakes

Condition: Brake Pedal is Low and Creeping / Falling to the Floor

Repair Procedure: Many owners have realized a low or creeping down brake pedal on their trucks. This condition may be corrected with several possibilities as solutions. Bleeding the brake hydraulic system because of air contamination. Adjusting the drum brake mechanism to correct the shoe to drum gap tolerance. Replacing a defective electro hydraulic dump valve because of a defective dump solenoid. In addition an approved Ford repair to increase the master cylinder bore diameter by using a F450 Super Duty master cylinder specific to the vehicle being repaired. Lastly, a new repair possibility exists involving the vacuum booster on a diesel truck. If the condition occurs after the vehicle has come to a stop, and no other brake concerns are found the condition is considered normal as the result of the vacuum pump replenishing the vacuum inside the power booster. Ford has developed a Zero-Loss Travel brake booster to correct this condition if the customer finds this condition unacceptable. This part is only for use on diesel-powered trucks P/N F5TZ-2005-CA it is not a service replacement, it must be ordered by the part number specifically, not the vehicle application.

Thus, this led me to submit another post "Diesel Master Cylinder" asking the general consensus on the bore diameter on the M/C for the diesel vehicles. Only one response.

I dug into my service papers and found a ford part number for a master cylinder that was replaced at the dealer years ago. I checked that number against what was supposed to be the original part number for the M/C and they didn't match.

What was the difference???

I took the original part number and cross referenced it in the NAPA web site and it returned the part that my local NAPA gave me when I replaced the M/C in December. This M/C has a bore diameter of 1 1/16" and was listed for a diesel powered vehicle.

I took the part number that I found in the service papers and did the same cross reference. This revealed a M/C with a bore diameter of 1 1/8" and was listed for a gas powered vehicle.

Now revert back to the above TSB. It talks about increasing the bore diameter of the M/C. It's all starting to make sense. Also, seeing Jeff Dodson's post about him using a F-450 M/C, which he drilled out the ports a bit, solved his problem.

I took my new M/C NAPA p/n 47-39634 and had them give me a new, not rebuilt, p/n 47-39635 M/C.

I bench bled the M/C with no foreign materials coming out of the bleed hoses, and installed it in the truck.

After bleeding everything, the pedal was nice and solid, just like the previous M/C. Here's the moment of truth. Will the pedal sink to the floor when the vacuum pump is supplying vacuum assist???

OF COURSE NOT!!!

The pedal performed just like it did before the rear steel brake line failed. I adjusted the rear brakes a little and off into the sunset I went!

If I never found the change in the M/C from the service papers, I would have went down to NAPA and gotten a F-450 M/C which would have been pretty much the same fix as Jeff Dodson, but I would be hesitant to drill out the ports from the start.

No misplaced brake lines. No problems with the brake hoses that were replaced two years ago. No air trapped somewhere in the lines after I've gone through a gallon and a half of brake fluid. No faulty master cylinders that contained metal shavings. No air infiltration from the threaded fittings. No problems with the RABS valve. I didn't have to replace the vacuum booster and vacuum pump. I never gravity bled the brakes either.

I had the same problem with my truck. Your pressurizing the ABS valve which is taking most of the pedal stroke to do it then it goes to your brakes. I just disconnected mine the ABS has not worked right in years anyway. Now I can face plant myself on the windshield. It was weird because it all worked before until I changed all the brake lines. Changed the M/C x2 Bled system x6 bypassed electronic junk (ABS Valve) x1 = success.