I have a 1993 Ford E350 7.3 diesel dually retired ambulance (built by National Ambulance Builders, which folded in 2000) and four-or-so months ago had the brake light come on, and the driver's side rear brake shoe was getting stuck and dragging, heating up the wheels and producing the infamous hot brake smell. Naturally, I took it into my local ASE shop that has a bay and lift large enough that can hoist the five-ton behemoth into the air. The pads and shoes were worn, so had those replaced while they replaced the self-adjuster lever in the drum that seemed to be the culprit.

This worked for a short while, but brake light came back on, so I dropped it off again. Other problems with the brake system were identified, so one-by-one, they replaced virtually every part of the system – shoes, pads, drums, rotors, RABS valve, proportioning valve, master cylinder (3x), power booster (2x), etc. At some point, braking performance all but disappeared, as the stopping distance basically doubled or tripled, wherein previously I could stop from 30 to zero (these are made up numbers) in 50 feet, it now takes 100+ feet! In addition, it feels like pumping the brakes three or four times (a natural thing to do when it feels like you aren't going to stop in time) actually generates less braking force with each subsequent pump. This makes driving a scary proposition, so I've opted to leave it with them until this is sorted out, occasionally taking it for nerve-racking test drives.

Of course, the brakes have been bled numerous times, pressure and smoke tests have been done pedal is high and hard, no leaks, braking is balanced and even at the wheels (slight bias to the rear from the proportioner) – all the stuff anyone can think of. The reason for the multiple master cylinders and power boosters is that after some testing revealed a defective master cylinder out-of-the box, they wanted to ensure properly matched parts (although they don't appear to need to be a matched set) and eliminate the possibility of a problem with either specific part, as those seem to be the likely candidates.

The suspicion is that the brake system was upgraded from the factory by the ambulance builder (presumably the power booster and/or master cylinder) and when they returned the old parts (now lost forever) they inadvertently got rid of whatever upgraded items that were necessary to stop the weighty beast, and just put in what the VIN number came back with (which would've been what Ford put in at the factory).

Research they have done on the subject has produced hints at things like installing large-bore master cylinders, zero-clearance boosters, caliper wear problems, TSBs for brakes in the same model but for other years that basically read as silent recalls from Ford (stuff done under warranty only if a problem exists), but these are guesses at best, don't seem to solve the problem. I called Ford, and they say that parts replaced during the ambulance prep done at the factory are associated with the VIN, and many ambulance manufacturers upgrade the brakes from there.

I am tired of them shotgunning this thing, and want some real facts on the subject. Has anyone got or had an E350/7.3 ambulance (or I suppose other heavy-duty application) of this era and can they definitively tell me what master cylinder/booster is in their vehicle? It seems to me this is the only way we are going to get this resolved. If anyone has an opinion that I am way off in presuming that it is the master cylinder or power booster, please speak up and let me know what you think the culprit is.

Thank you.

 

You are looking for the correct combination of the mc/brake booster for a 93 E350 right? Maybe I missed it, but its convention vacuum breaks, not hydroboost right

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And the light coming on usually means there's low fluid, or in this case, low vacuum, have you swapped out the vacuum pump on the engine? My guess is that will fix your problem

 

Nope, didn't miss it. I forgot to mention it. It has a conventional vacuum system, and the pump remains steady in the shop - they said that they even rigged a tee and taped a gauge to the windshield to road test it, including testing various points in the system. If I recall, they said something like 22 inches (does that sound right?).

 

Pedal goes to floor?

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The RABS valve will likely be the problem, if it wasn't bled or bled correctly, it won't let the system pressurize properly (Air is non-compressible in a hydrolic system) but it also sounds like there is a vacuum leak, I would change my booster just to be sure.

 

I would've guessed the vacuum pump too since everything else has been changed 2-3 times. Maybe try calling a uhaul repair shop, I remember when I turned wrenches at our local one in the mid '90's the EL's where ambulance package econolines so they may know what to use. I've also seen online that some of the repair facilities for uhauk are selling parts now, maybe a place to get parts from.

 

Running out of possible parts here

 

I too think something else is awry here ... Rabs ... never heard of it ... I would swap over to hydro's and be done guessing ... also .. truck running while bleeding will help force air and volumes of fluid and eliminate air cavity issues ....

I used to work on motor homes . this was how I did it ... running , foot on pedal and slow crack on the bleeder with a 1/8" clear line back into a bottle... worked real good