front brakes overheat on first run of the day--why?

[DougJ]

I had my Ford chassis 1990 Winnebago Elandan in a sick bay at a Ford dealership that services (Ford) motorhomes for a power steering box problem, and while there I asked them to check my front brakes because they overheat (350F to 470F on the rotor) within the first 30 to 40 km run of the day at 100km per hour. 

They road tested the rig (learned today a measly 5km at city speeds) and said it didn't overheat; I said I could have told them it wouldn't.  They suggested new calipers and pads because the existing ones showed signs of overheating.  I asked them to check the rotor to be sure it had no warp in it--it didn't.

Picked the rig up this morning and told the service adviser I'd call in thirty to forty minutes to tell them either no over heating, or yes, it is overheating.

At the start, the rotor (with its new calipers and pads) was at ambient temperature: 75F. 

I drove 4km at 100km/hour then pulled into a rest stop and measured the temperatures of the rotors: both at 120F.

I then drove 27km at 100km/hour and pulled into a weigh station and measured the temperatures of the rotors: R at 341F and L at 323F.

Shut down the engine for ten, but not more than fifteen minutes while I called the Ford dealership who said, bring it back.  I then drove 39km back to the dealership, of which about 32km were at 100km/hr.  The temperatures at the dealership were R at 130F and L at 132F.

These data fall within the pattern I've observed over about two years and which I cope with by stopping after the first 30 to 40km and shutting down for a short period of time.  I'd say that 90% of the rotor temperatures I observe range between 350F and 480F--and all this with virtually no braking, just 100km/hour on straight and level highway.

I have noticed, but not tested to demonstrate reproducibility, that if I start the engine and let it run until the radiator temperature gauge is in the normal range, and then shut down for a short period of time, and then run for the first 30 to 40 km that the rotors do not overheat.

One more observation.  On the first start up of the day, the brake pedal has little play and is very firm; but after that first shut down, the pedal has more play and on braking there is what I would describe as a normal softmess when pressing down on the pedal.

Looking forward to your thoughts because I have an appointment with the techie on Tuesday.

Ciao,

Doug

 

[Karl]

Doug,

About the only things that cause dragging disc brakes are corroded or dirty guide pins, dirty/rusted rails, or sticking pistons. Since these have all been replaced, we are left with few other possibilities - brake pedal not returning completely (weak return spring), riding the brake pedal (you'd know if you're doing that), or, and this is the most probable cause, the shuttle valve in the master cylinder is sticking. Because you said it goes away after running for a while and shutting off the engine, my guess is that the engine heat is freeing up the valve. Try this: With the engine cold, take a 250 watt heat lamp and aim it at the master cylinder to get it good and warm. May take 15-30 minutes. Then drive your normal distance and take a reading. If it's normal, you've probably found the problem, but before replacing the it, have your service guy do a COMPLETE power flush, refill, and bleed of the whole system. Could be contaminated fluid causing the problem (brake fluid is very hygroscopic and water makes a lousy lubricant). If that doesn't work, you'll probably need a new master cylinder. 

 

[DougJ]

Thanks for the reply, Karl.  I'm pushing for the master cylinder, the proportioning valve, and the rear flex hoses to be replaced--I'm not convinced that the rear brakes are sharing the braking load, although I'm told that with the brake engaged, they can't turn the wheel by hand. 

However, I can't yet find a dealership that steps up to the plate to take my money

Ciao,

Doug

 

[Karl]

Doug,

I'm told that with the brake engaged, they can't turn the wheel by hand.

BIG DEAL! If their brake test is nothing more than trying to turn it by hand, run; don't walk, away from them! I'm guessing you have drums on the rear. Second best way to test them is to block the front, jack up the rear, and apply power with the engine to see if they work. They are 'kind of' self-adjusting, that is they will adjust when you back up and apply a reasonably heavy pedal to them. Most of us don't back up an rv often enough or apply the brakes hard enough for that to happen. If not, the adjusters will quickly build up rust and/or corrosion and won't work at all. If they're too far out of adjustment, you can get an adjusting tool and turn the starwheel by hand through a grommeted hole in the backing plate. The proportioning valve might be working properly but the gap between the shoes and the drum is just too great for them to be effective. That could be part of the problem, so I'd have them pull the rears and check the shoes, drums, and adjusters. That doesn't, however, explain why they seem to work after you run for a while. I'd still try the heat lamp thingy.

 

[DougJ]

Hi Karl,

Thanks for your further reply.

The rear wheels on my rig are disc brakes as are the front ones.

Ciao,

Doug

 

[Karl]

Doug,
You're lucky. No adjustment necessary, but I'd still find someone else to do business with.

Humor me - try the heat lamp

 

[Chet18013]

Doug,
You're lucky. No adjustment necessary, but I'd still find someone else to do business with.

Humor me - try the heat lamp

Doug, I agree with Karl. Sounds like moisture in tht system, which is expanding the fluid as you heat the system up.  If you do replace the master cyclinder, besure to insist the the system be COMPLETELY flushed and the fluid replced.

Chet18013

 

[DougJ]

Hi Chet & Karl,

Sounds like moisture in that system, which is expanding the fluid as you heat the system up.  If you do replace the master cylinder, be sure to insist the the system be COMPLETELY flushed and the fluid replaced.

I think you're both right.  My problem is that futzing with things and tools is way outside my competence zone, and what makes it worse I have no interest in trying to bring it within my competence zone. 

Having said that, I am wondering why if the heat cause the expansion and hence the brake pads to engage, why does shutting down the engine cause that situation to relax?

Anyway, I'm going to work on persuading the techies first to flush and replace the brake fluid, and if that doesn't do the trick, then to replace the master cylinder.

Ciao,

Doug

 

[Chet18013]

If my memory serves me right, there are now 3 different types of brake fluid on the market. They have different performance and temperature ratings. Make sure that they select the right one, that meets the specs for your vehicle.

Chet18013

 

[Karl]

Doug,

Having said that, I am wondering why if the heat cause the expansion and hence the brake pads to engage, why does shutting down the engine cause that situation to relax?

My theory about the heat is that you have a sticking valve in the master cylinder due to corrosion and/or water. When you're driving, air flow under the hood keeps it relatively cool; then when you shut down, the engine heat heats it up and frees it. Moisture in the brake fluid lowers its' boiling point and allows (causes) bubbles to form and gives you a SOFT pedal; not a hard one.

 

[DougJ]

Hi Karl,

My theory about the heat is that you have a sticking valve in the master cylinder due to corrosion and/or water. When you're driving, air flow under the hood keeps it relatively cool; then when you shut down, the engine heat heats it up and frees it. Moisture in the brake fluid lowers its' boiling point and allows (causes) bubbles to form and gives you a SOFT pedal; not a hard one.

Makes sense to me .

Ciao,

Doug

 

[DougJ]

to keep you in the loop on this matter:

To give the techie with whom I was to chat somethings to think about, I wrote a detailed account of the presenting symptoms, added some other details of maintenance that I thought might have a bearing on the primary issue, and then set down my suggestions (garnered from this thread and a thread on another forum) in the form of seven questions.

We did speak and the techie said he had no idea what might be causing the problem; had never run across anything like this.  He asked me to bide my time until Thursday when the manager of service is back on duty.  He thought that he and the manager should be in contact with Ford's techies for ideas.  I pressed on the issue of replacing the master cylinder and he said he'd get that priced out for me.

So, I now wait until Thursday afternoon--at least this dealership isn't into bullsh**ing me and seems willing to make an effort to uncover the real problem.

Ciao,

Doug

 

[Karl]

Doug,

I don't mean to rain on your parade, but typically service managers are just service writers, and know less than the guys who actually do the work. But it's a good start. If he can get the zone manager involved, you might get a successful resolution.

Good Luck!

 

[DougJ]

I don't mean to rain on your parade, but typically service managers are just service writers,

My interpretation of this, Karl, is that the techie isn't allowed to go outside of the dealership without the manager's OK.  I have to remain hopeful because at this point this dealership is taking more action than the others have.  The others have sent me on my way (mind you I was in the middle of a trip on those occasions) ready to burn up another set of calipers and pads .

Ciao,

Doug

 

[DougJ]

A little updating is in order on this matter.

The service manager for the Ford dealership is back on the job and on Friday I heard from the service ad visor the service manager thought we should replace the master cylinder.  I don't know what his thought process that got him there--the symptoms I described plus one of my suggestions being to replace the master cylinder, or that he's seen this problem before and knows that it might most likely be a problem with the master cylinder.

Part was supposed to arrive on Saturday and they'd install it on Saturday, and I could get the rig back on Saturday.  But on Saturday there was not part delivered; Monday is a holiday here in BC; and, they don't know if that will throw off the delivery schedule meaning no part until Wednesday.

I think I'll chat with them on Tuesday and ask them to reset the brake pedal (piston stuff) to factory specs as part of replacing the master cylinder.

Ciao,

Doug

 

[Just Lou]

Doug,  you haven't given us an update on this post in a while.  What's the latest?

lou

 

[DougJ]

you haven't given us an update on this post in a while.  What's the latest?

Hi Lou,

The latest is as follows:

1.  Following my demonstration that replacing the calipers and pads did not solve the problem--360F, + / - 20F, on the front rotors after 30km at an average speed of about 95km/hr, the techies and service manager have consulted among themselves (and perhaps with others) and suggested to me that the master cylinder should be replaced.

2.  I agreed. They replaced the master cylinder, but that did not solve the problem.  So then they suggested that the hydroboost should be replaced.

3.  I agreed, pointing out that I had done that less than three years ago.  But, that did not solve the problem.  In fact, what the service adviser told me is that the pedal is not recovering to its propoer position by one half of an inch (and I've no idea where they are measuring that).  So,

4. they have ordered another hydroboost in the expectation that the first one was not up to spec.  That latest hydroboost was to be installed today--but as of 1630hrs local, I have not heard from them. 

5.  When I asked the service adviser what their next move would be if the replacement hydroboost did not solve the problem, she said they didn't know other than they would be scratching their head.  Seems to me that if it remains a problem the conclusion has to be that either there is a "recovery spring" missing or grown too weak, or something is binding between the pedal and the master cylinder.  But, understand:  I don't know diddly squat about these kind of things.

If you have comments or suggestions, please share them with me .

Ciao,

Doug

 

[Just Lou]

Doug,

I was about to suggest that some of your symptoms (especially your statement that when you warmed up the engine before driving that the rotors did not overheat) sounded like maybe a vacuum booster or vacuum related problem as most older engines with emission controls on them will play games with ignition timing, etc.., during warm up and some are still vacuum controlled.
Thus my attempt to connect the brakes to a possible vacuum problem.? just an idea...

You threw me a curve though with the hydrobooster.? I understand the theory but I'm not at all experienced with those.? If not a mechanical problem then you would be looking at a possible hydraulic problem.? I think most of them are driven off the power steering pump with proportionate boost to front and rear brakes.

Was this an after market conversion from vacuumboost to hydroboost or am I still living in the dark ages?

I just did a little power reading and learned (as probably most everyone else already knew) that hydroboost brake systems have been fairly  standard on trucks and RV's for some time.  Back to the drawing board for me.....

 

[DougJ]

I think most of them are driven off the power steering pump with proportionate boost to front and rear brakes.

Was this an after market conversion from vacuumboost to hydroboost or am I still living in the dark ages?

Let me say again, Lou, that almost all of this stuff is in the realm of the black arts for me.

However, I can confirm that my braking system (including the hydroboost thingy) is driven off the power steering pump.  On one trip I was in the BC mountains when the power steering pump failed, leaving me with no brakes and an incipient hernia every time I had to negotiate a curve.  Fortunately it was only about 10km (IIRC) on relatively flat highway to a resort where I could pull off the road and get to a land line (there being no cel service).

To the best of my knowledge, my hydroboost system is not after market--it was the OEM approach to power assisted brakes on this 1990 Ford chassis.

Given my ignorance of these systems, I am thinking that if the new hydroboost doesn't solve the problem, then the cause of the problem, i.e., the brake pedal not retracting fully (and assuming I've understood that to be a presenting symptom at this point), must be some form of binding--a tube carrying a cable that is pinched, for example.

But do you have any further ideas?

Ciao,

Doug

 

[Karl]

Doug,

You didn't mention the hydroboost before - shame on you

There's a rod from the master cylinder to the hydroboost unit to actuate it. If it's too short, you'll get too little braking. If too long, the fluid will not return to the reservior and the brakes will lock up or drag. This rod SHOULD be adjustable. Can you take off the cover of the MC and look inside? If so, start the engine and have someone gently push on the brake pedal. You should see fluid begin to spurt out of the return port almost immediately (it won't create a geyser if the pedal application is gentle). That's what you want to see. If you apply full pedal and don't see the fluid returning, that would indicate an incorrect rod length setting. Just replacing the hydroboost unit without setting the rod adjustment will do nothing for your problem.