Retired Ambulance Stopping Distance Problem - Diesel Truck Forum - TheDieselGarage.com
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post #1 of 8 (permalink) Old 02-13-2014, 12:21 AM Thread Starter
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Unhappy Retired Ambulance Stopping Distance Problem

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.
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post #2 of 8 (permalink) Old 02-13-2014, 02:06 AM
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Re brakes

What model is this ambulance,Daimler or crestliner?i have 5 years experience on both models.I am heavy truck and equipment mechanic.Question?is your vacuum pump working properly,meaning when hose removed from brake booster is it strong vacum.Low vacuum,hard brake pedal to a point in which you want to use two feet to push pedal and still poor braking.I need to ask questions in order to narrow things down!
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post #3 of 8 (permalink) Old 02-13-2014, 02:21 AM
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Re brakes

Keep in mind,brake light only comes on when there is a pressure difference between the front and rear brakes.did they happen to look at the rear brake calipers upper retainer bolt isn't being bend over by the leaf springs sitting on the bolt when unit is on the ground.Easy to see if you climb under and look at the upper calipers bolt.If leaf springs are weak and saging ,they will bent the slider calipers bolt over,thus calipers not sliding properly!!!!!
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post #4 of 8 (permalink) Old 02-13-2014, 05:15 AM Thread Starter
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It was built by National Ambulance Builders out of Florida, but I'm not sure if they were related in any way to Daimler or Crestliner. The vacuum pump apparently holds 22 inches steadily (good, I assume?), including on a road test that they rigged an inline tee (and tested several locations) with the gauge taped to the windshield.

The brake pedal is hard. I would describe it as essentially no assist at all - you've basically got to apply emergency braking pedal force to get it to roll gently to a stop. It used to be a good stomp on the pedal would shove you into your seatbelt and send stuff flying off the shelves in the back and bring the rig to a decisive, if not violent halt. Not anymore, though. You can push like you were going to jam the pedal through the floor boards, and it gets around to slowing down.
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post #5 of 8 (permalink) Old 02-13-2014, 12:50 PM
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re brake pedal

did they check the flow through the front brake flex hoses.Hoses are known to collpsed at the retainer braket thus restricking brake pedal flow through the hose.I have many times spread open the retainer clamp that holds the brake hose inplace.IE screw drive and spred holder and presto full flow.What happens rust gets between hose holder and squishes hose and restricks fluid.When brakes applied should have heavy flow out of the bleeder.If the pedal is hard,when you press your brakes do you loose power steering?
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post #6 of 8 (permalink) Old 03-03-2014, 03:19 PM
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It could b a line restriction, not letting the vacuum to pull the shoe after pressurizing.
If it is only one wheel grabbing and not releasing then between wheel cylinder and first
Junction in brake lines....Marc covered this, but I was wondering if there just might b a dent
In one of the steel lines
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post #7 of 8 (permalink) Old 03-03-2014, 07:33 PM
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Couple things.

First woud be booster adjustment. If the adjustment rod is set up incorrectly you will basically have manual brakes, super hard pedal, no stopping force.

Next would be master cylinder bore size. The larger the bore is, the higher the effort will be at the pedal. You may have a master cylinder for a single rear wheel application or for a 1/2 ton, and you likely need one for an E/F-SuperDuty (E/F-450) as the brakes were very close from application to application back then and you may have received mis-boxed parts, multiple times even. Another problem is parts manufacturers consolidating master cylinder part numbers on the basis that 11/16, 3/4, 13/16, 7/8, 1, and 1-1/16 are all pretty close and instead of keeping 6 part numbers if 13/16 and 1-1/16 are the most popular, they will only keep the two most popular and merge the other applications with the remaining sizes.

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post #8 of 8 (permalink) Old 04-05-2014, 04:10 PM
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drive ambulance with brakes on for a while. shoot with temp gun, cold brakes aren't working. Then you can track down the problem.



If none are hot take it to another shop.

Or just take it to another shop.
Sounds like a vacuum problem to me too.

Last edited by testlight; 04-05-2014 at 04:13 PM.
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