I have a'94 F350 crew cab dually with the 7.3 IDI engine.
last year I had both front brakes completely done, nothing to the rear. I was driving on day and lost all my brake pedal. Went to the floor and barely stop the truck.
I changed the master cylinder due to the fact that it was very old, may as well. The booster works , I replaced the vacuum pump last year,
My mechanic has been replacing any bad lines , but still no brake pedal. Engine off, bleed them.... Engine on, no pedal
Help....

 

inspect your rear wheels for leaking wheel cylinders... IF you have a Mechanic working on it and he can't bleed the brakes to give you Pedal you have a Leak somewhere.

 

BTW IF you have never changed the Rear shoes then it's probably High time to inspect them.

 

Is there any pressure at the pedal at all? If the wheel cylinders were leaking you still had a little pressure to push the fluid out. Plus the backs of the wheels would be wet from the fluid. If no pressure at all I would think you have a problem between the pedal and master cylinder.

 

I've seen them several times where the rear wheel cylinder leaks air but not fluid. Which is weird because normally when they leak fluid they still work good. Changing them always solved the pedal problem for me when it would get hard but soft once engine was on

 

I hope you aren't paying him. Find a new mechanic.

 

Thank you kindly fellas for your replyies.
I myself am not a mechanic but have worked on vehicles large & small all my life, including my road tractors.
I have been though multiple surgeries on my back & hips which has disabled me and greatly reduced the amount of work I'm able to do anymore.
I did replace a few bad break lines and the master cylinder as I said. I thought about the seals being bad, but there's no fluid anywhere. I suppose I should have them replaced, but I really never heard of them going bad without leaking

 

Even if the rear brakes were blown wide open there should still be good brake, front/rear are separate systems. The going nearly to the floor before stopping is indicative of a failure of the _front_ brakes rather then the rear.
But either way you should be getting some brake pressure so either you got a bad new master cylinder (possible) and/or your mechanic is an idiot (also possible).
Also make sure you got the correct master cylinder, there are different ones depending on the GVWR of the truck.

 

straight from Wagner... I have seen this dozens of times too....

"A “no brake pedal” condition can be encountered after a new master cylinder is installed, leading the technician to believe that the master cylinder is defective. This is likely not the case; the condition can be caused by the piston sticking in bore of the master cylinder during the bleeding process when the brake system is manually bled. The brake pedal will go to the floor and subsequent efforts to get a satisfactory brake pedal will fail. A good indication of this condition is lack of fluid flow from the brake bleeders."

 

I'm no expert but I had this condition happen in my 94 when it was still running. During the time it was(still is) down I swapped the axles from the 94 to my 88... Which had perfect and very responsive brakes pre-axle swap - this changed when swapping specifically the rear end. The problem then transferred from the 94 to the 88 where I would get no brake pedal but pumping as I decelerated while downshifting resulting in a perfectly firm and non-fading pedal after a few presses. After some research I found people stating that excessively worn pads or drums could cause this type of symptom where your brake pistons need to expand more than a single pedal press can allow but by pumping they don't have time to fully return and get pressed out more to the point where they engage.

Have you tried pumping your brakes with the engine running? Does pumping firm them up? Do you get fade on holding the pedal if they firm up?

As mentioned..... Fade would definitely mean there's air getting in somewhere. Pumping and firmness would lead to either new brake components needed or possibly just an adjustment on the shoes to get them closer to the drum surfaces. I believe there is a "star nut" or called something like that for this - accessible from the back side of the drum.

 

Again I appreciate the replyies fellas. I'll have to wait till Thursday to talk to the shop in town.
Thanks & Merry Christmas

 

BUG em on Xmas Day

Merry Christmas to you too, hopes it is something simple.... I know Disabled ... I am too... but I refused to let them Mess with my back... trust me they wanted too. and I think I would be worse off IF they did... BUT your mileage may vary.. depends on individual condition mine was from Nam back in the 60's

 

I know its been said that the front and rear could be independent but I would honestly check (or have the shop check) what I said. I'm not stating that anyone here is right or wrong but upon adjusting the star nut (is referenced in the service manual) I gained adequate braking again. You may be experiencing the same phenomena I have - which led me to reply to your post. Now, this depends on if you get fade or not while the motor is running... I have a pressure bleeder that I use for my and other's braking systems that send pressure/fluid through the master and maintains a determined PSI on the system (easy one-person bleeding for under $100) and after many hours spent attempting to relieve the problem the adjustment helped significantly to return natural brake feel. (thought I know I need to replace components soon). Maybe something to relay to your boys at the shop.

Thank you, sir! And Merry Christmas to you and yours!

 

you can actually Gravity Bleed the brake systems on our ford trucks.. our trucks do not have a proportioning valve... the front and Rear are separate circuits.

and yes the back brakes could be way out of adjustment, easy to check just spin the wheel. They are self adjusting but that don't always work... I still think you need to inspect the rear brakes if you have never done it... especially since you have a NO BRAKE Issue

there are several size shoes on the rear too my 93 F250 uses 3.5" shoes it's a code 69 axle.

 

Also make sure they are bleeding the RABS (Driver's side frame rail near the back end of the motor)... Forgot about that... A professional shop should know but.... Just saying.. Just in case since no ones said....