Brakes applying by themselves

[Mont91]

This is on a '94 B series school bus. When going down the road the brakes slowly apply themselves. This started after a truck repair shop put a new hydraulic booster on. We just got it back again yesterday and they are still setting up. It took about 60 miles this time before they started smoking after supposedly being fixed. I was not driving. This is my main bus to use. When I use it I rarely get more than 10 or fifteen miles before I have to stop, shut it off, then restart to go on down the road. My route is 70 miles long so this gets to be a pain. I can tell when they're setting up because the brake pedal slowly looses its travel, when the travel is gone your done moving. They usually just apply a little to mostly, this of course causes sever brake wear. Any ideas. The nearest repair shop is 100 miles away so it is not very easy to get it back to the shop.

 

[towcat]

to me, it sounds like a bad rebuilt hydro. The valve inside is allowing pressure to build up slowly when it should be in bypass mode. Get the shop to change it out for another, pay close attention to the master cylinder pushrod length on the master side and if they don't know or won't do it right, bite the bullet and take it to another shop. What you do for a living, there's too much at stake.

 

[Mont91]

Forgot to mention this is the second booster and new parking brake manifold and control valve. It has hydraulic parking brakes, not the kind that locks the brakes with the pedal pushed down but the kind that has a seperate can on each rear wheel. It pushes the brake pedal back to the top when the valve lever is flipped. This does not seem right but one of the other buses, a B 700 does the same thing. The service brake booster and the parking brake use the same hydro pump for pressure. It would be nice to know how this system functions and how it functions pproperly. The shop doesn,t know what to do next. Our regular shop won't mess with these type of parking brakes.
P.S. Where the push rod goes through the firewall into the booster is so hot after a run it could burn a tender finger. Can't find an adjustment point on the rod yet.

 

[Magoo]

One good turn deserves another. Computers I cant help but brakes? Maybe. Towcat is right. If you had the booster changed out twice than suspect the actuator rod. I am not familiar with the 94B but there are only a few versions. There is the pedal rod (from pedal through firewall) and then the actuator rod between the booster and the master or hydro which applies the valve assymbly.. Sounds like the actuator rod is too long. Keeps residual pressure on the system and each push of the pedal stores more pressure. I had an old diesel caddy come in one time with a similar problem. As long as it was running the pressure from the pump held fluid behind the valve and would not let the valve return. Turn the motor off and pressure released the valve, and you would get normal breaks. Start up head down the road and after a few pumps of the pedal you were locked up again. Good luck. Hope this helps. I owe you one. Just a thought here. Heat on the pedal rod is a new one. If it is hot from transfer of heat from brake hydro unit then hydro must be near glowing. Can you smell burnt oil?

 

[Mont91]

I would like to find a fluid flow chart for this system. Is the actuator rod changed or supposed to be checked for propper adjustment when the booster is changed. This problem showed up after they changed the original booster because they said it was leaking. The other thing I don't under stand is why the front brakes lock up when the parking brake is set. I set the parking brake while on some ice(no shortage of this) at a crawl to test what worked. The front end slid side ways so I am assuming the front tires locked. Why would this happen and why does the bake pedal push my foot up when I set the parking brake and let it down when released. If the booster is calling for pressure all the time would this heat up the oil?

 

[Magoo]

I would like to find a fluid flow chart for this system. Is the actuator rod changed or supposed to be checked for propper adjustment when the booster is changed. This problem showed up after they changed the original booster because they said it was leaking. The other thing I don't under stand is why the front brakes lock up when the parking brake is set. I set the parking brake while on some ice(no shortage of this) at a crawl to test what worked. The front end slid side ways so I am assuming the front tires locked. Why would this happen and why does the bake pedal push my foot up when I set the parking brake and let it down when released. If the booster is calling for pressure all the time would this heat up the oil?

If the pedal raises when you apply P brake sounds like it is charging the the whole system. if I understand right the park brake is actuated by a lever that "without" applying the pedal first, applies pressure to the rear brakes? That would be new to me. What I have seen are "independent" hydraulic park brakes that do that but they use there own master and slave cylinder and the park mechanisim at the rear axels were stand alone. The other more common type shared the hydrulics with the service brakes but the lever would "trap" pressure to front, rear or both axels. This system requires you to apply the pedal then apply the park lever. There are many manufactures of these and the cheaper ones tended to bleed off over time. I used to work on these on the older medium duty fire trucks without air brakes. They tended to push the pedal up when the park lever was engaged now that I think about it. Now that I think about it if this is the system you have it could be that the park brake unit is accumulating pressure till the system is locked up. Just a wag but it could happen. The park is between the pedal and the brakes so all fluid passes through the park unit on its way to the brakes. This is all assuming you have this type of system. By the way is there no electric booster? Can you snap a picture of the booster and park system and post it? To answer the booster actuator rod question, yes they are adjustable. The booster it self has has a spring and diaphram that has a fixed ammount of travel. The actuator rod sits in a piston and has to have free travel between the piston "inside the booster" and the valve assymbly in the master. I learned from expierence to quickly measure these before I install since the remanufactures use parts not necessarily desinged for a specific application. Good luck. Is this a scool bus? For kids? In cold country? You want to borrow a a big stick or some other form of attention getter and take it to the shop for a quiet conversation with the mechanic?

 

[NJKen]

This really sounds like the pushrod that comes out of the hydrobooster is too long and not allowing the master cylinder piston to come full back to its stop. This will block ports in the master cylinder and can cause everything you are experiencing. The reason the brake pedal pushed back at you is because when the parkinb brake is actuated you are actually letting pressure out of those 2 cans on the rear axle. There is a big spring in that can that drives a wedge into a modified wheelcylinder. This causes the shoes to press on the hydraulic wheel cylinder that is connected to the master cylinder. If the ports in the master cylinder are blocked because the piston is not at rest the fluid has nowhere to go. That could be why you feel it fighting you in the brake pedal. The truck we have with this system do cause a little pedal rise when you engage the parking brake but it is not excessive. A quick test would be to unbolt the master cylinder and put a whaser over each stud then stick the master back on. This will space it a little further from the booster and should correct the problem. If that fixes it the next step is to figure out how to adjust that rod.
Best of luck
Ken

 

[Mont91]

New Wrinkle. But first a reply.

Hey Magoo, your idea sounds good. Haven't got anywhere by . And there is an electric booster motor but I never hear it accept when the key is on and engine off. Will work on the picture.

Thanks NJKen for the description of function. Hopefully will have time today to try the washers.

towcats description ofa slow build up seems right most times, other times seems like it happens imediatly. Every thing seems to come back to that actuator rod.

Now for the new wrinkle. When sitting still idling about every 30 seconds the parking brake manifold, the part under the hood with all the hoses going to it that controls the parking brakes, goes kerklunk. When sitting still I can hear it, so I assume it is happening while driving. When setting the parking brake the manifold and the booster/master go kerklunk. I can also feel this in the pressure supply hose from the pump. You don't want to hang on to this hose very long because it is hot. I think this has been happening all along, just didn;t notice it.

 

[Magoo]

Ok. Hyd. fluid when circulating in a properly design system gets warm. Actual heat depends obviously. If the fluid is under too much pressure or not circulating it gets "hot". If the acuator rod (booster to master) is too long, and the fluid cannot circulate through the pump it will get hot and build pressure. (presure to the brakes is also in existence on the other side of the valve assymbly) causinbg brake drag. I can only guess the kerklunck is when the pressure builds so much that is has to go somewhere and my guess is it may be overriding pressure behind the valve and slamminng it shut. Either way this thing needs to be taken out of service untill fixed. Too much at risk here. Gut says actuator rod.

 

[Mont91]

Update. Blew seal or someting on right front caliper last morning of school, left for vacation hours later. Got back today and it is still sitting where I left it. State bus inspection due by end of January. Wonder what they are going to do. Wonder what I am driving Tuesday.

 

[Mont91]

I should be writing to say the problem got fixed. No such luck. They apparently fixed the original problem by accident, but created a new problem in the process. Now the front calipers bind or sieze when they are used. On a test drive with the Bendix factory engineer the front brakes became extemely hot and after jacking up the front axle the tire could not be turned, this was done without shutting the engine off. The bleeder screw was opened but no apparent pressure present, a bar was used unsuccessfully to spread calipers. When the calipers cooled they released on thier own. The shop is trying to find a new set of calipers because the rebuilt calipers are apparently the problem. New calipers are not made any more so this may take awhile. One of the delays in solving the problem is that the original head mechanic was fired and didn't pass on all of the information regarding the history of the problem and what had already been done. The shop figures they have about $7,000 invested in this bus.

 

[Mont91]

Another set of rebuilt calipers, same problem. They can not find a new set. I think it would be cheaper to put a differant design of axle from a wrecked truck under it and call it good.

 

[abbamover]

If I might add a little info.
I have the same system in a 89 F800 and have also had problems with it.
Replaced the rear brake assm on both sides from Ford, the hydro-boost assm as new, the pump , front calipers, rotors, drums, etc.
Total in parts was over $ 2500.00 CAN.
Ford knows there is problems with this system.
It was designed by Lucas of England fame.
Seems to always leak in the rear cylinders, not during use but when sitting overnight, can tell during the circle check from streaks on the tires.
After 6 months the seals in the rears started leaking and I had trouble from the start with the hydro max unit.
There are 2 units for the system, one with a 1.75 inch bore the other with a 2.0 inch bore and using the wrong unit will give a over pressure in the system similar to the brakes being on all the time.
The unit I got was supercided to the 2.0 inch bore with a few mods from the factory.
I should have checked the valve unit that contols the parking brake, not the actual unit that has the yellow knob, but the one that is I think called a HR-1 ? Maybe the seals inside are rotten ?
I also get the kerchunking or a cycling every 20 seconds felt through the pedal.
My problem is different, I have a hard brake pedal like no pressure is going to the booster ( checked and it is ). Only sometimes do I get the boost effect.
I talked to a mech at Ford ( most don't care for this system ) and he suggested replacing all the seals in the system and using Castrol or Lockheed brake fluid, NOT Dot 3 .
Seems the normal fluid I've been using is eating the seals, got black fluid after 8 months.
Only thing I didn't check when rebuilding the brake system was the hr-1 parking brake control unit. The manual states it is to be replaced NOT rebuilt.
If you can find a similar good working system from a trashed bus, I'd pull everything and use it or switch to air brakes, much more user friendly.
My 2 canuk pennies.

 

[Mont91]

Well another month and still no bus . Can't find a set of calipers that won't sieze when used. The school won't take the bus back until fixed and the shop won't change anything from factory issue because it is a school bus. If it was anything else a differant axle with a differant design caliper would be put on. The school has desided to put a used rear axle under my last bus and start using it again. It spit the pinion bearings after a new rear spindel was welded on. It is a '93 chevy 1 ton diesel van with 200,000 miles. After the one in the shop this is my favorite bus. My current bus is an '88 E350 with a 351W C6 with 200,000 miles. I put about 25,000 miles per year on a bus. All of it dirt and gravel. I get the buses that are pulled off other routes because their wore out.

 

[Mont91]

My bus came home.

They finally put a spacer between the booster and master cylinder, mostely eliminating the problem. Rebuilt and readjusted the rear brakes, supposedly works fine. Seems the push rod was to long and held pressure against the pads over heating thus jaming the pistons in trhe calipers.

Does this sound familiar? It is really to bad I didn't get a chance to do this on my own before it went into the shop. It only took 8 months for them to try what we suggested.