Community Forums

cimplexsound · 12-11-2015, 09:54 AM

My brakes started acting up on my Tioga recently. My mechanic and I have been through thick and thin trying to figure out what could possibly be wrong. After I drive it about a mile the brakes will freeze up and I can't go anywhere. First symptom I can't get it past 25 MPH, then I have a hard pedal, them I'm stuck on the side of the road with frozen brakes. We're already $600.00 into repairs. We both think the master cylinder is shot. But he is checking the brake booster just to make sure that is not a problem too. My mechanic had to come out and rescue me twice yesterday. By loosening the mounting screws on the master cylinder to relive pressure on the system we were able to get it moving again long enough to get it back to the shop for repair. I never had a problem last winter. This big beast got me to Arizona and back without any trouble. And she runs great. It's just the brake problems. Napa Auto has the parts if that is indeed the problem. I am anxious to get out of here before the snow flys. So what do you think?? Everything else is in perfect working order. What could be wrong???

ChasA · 12-11-2015, 10:15 AM

When was the last time your brake fluid was changed? I've heard of this before. The fluid absorbs water and the the heat of the brakes boils the water creating steam, which increases pressure even more.

Jim_HiTek · 12-11-2015, 10:23 AM

I've also heard of this type of symptom and in that case it was older hose that the inner layer would age out and a flap of rubber would drop down into the hose after the RV'er had applied brakes a few times. It was temp sensitive so it appeared random. It would block the return flow of fluid so the brakes would stay applied.

Owners would usually just replace all the brake hose throughout the RV because there were bad batches of hoses produced over the years.

It would help if you told us what year, style, and model of RV you have when you post about a problem. Saves shooting in the dark somewhat.

Elf111 · 12-11-2015, 10:26 AM

Has your mechanic changed the flexible rubber hoses in the brake lines? These go bad after years of service and can cause the problems your having.

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spyderRV · 12-11-2015, 11:13 AM

When a master cylinder is fully disengaged, fluid can free flow through it. When you press the pedal, it closes these passages and develops pressure which is fed to the wheel cylinders. I would guess the linkage is not returning to fully depressed or the master cylinder itself is not fully disengaging and blocking the bleed holes.

twinboat · 12-11-2015, 02:49 PM

Do you have a vacuum booster with some linkage between it and the firewall.

The linkage could be binding holding your master cylinder from returning.

Had it happen on an old "80" car.

TQ60 · 12-11-2015, 08:07 PM

First just do simple things.

Bleed them first and use a hose on the fitting so you can catch the fluid and check it.

Some of the hoses between the caliber and frame go bad and clog up.

They will allow fluid to caliber as there is great pressure but they do not retract as no pressure there.

Next try to get them to stick then have wrenches handy to open lines at master cylinder or wheel to see where pressure is residing.

Check each wheel to look for signs of heat as that may indicate a sticking wheel.

You will need to get clever if the pros have not fixed it.

Elf111 · 12-11-2015, 08:36 PM

Just another thought, have you checked your e brake? Invest in a laser temperature gauge a short ride around the block and start checking for temperature differences locked brakes will get hot fast. The gauge is a handy tool to check tire temps while traveling.

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TQ60 · 12-12-2015, 08:56 AM

Following that...

Do get an ir thermometer as they are cheap and handy but perfect for this.

Take a short trip around the block then scan the hubs and if you can crawl 1/2 way under check each drum.

Do it quickly after stopping as they cool so having a youngster handy works here.

Looking for hot spots but check axle ends to compare.

Both fronts should be close and both rears should be close.

First time is short and for practice but you may get something.

Next make a longer trip with lots of brake action to get things moving but keep close to home, maybe still around the block so to speak but just pump brakes.

Then drive a short distance with minimal brakes then slow to to where stopping is just a tap si no heat from that.

A dragging brake will now be warmer than first readings.

FIRE UP · 12-12-2015, 11:30 AM

Quote:

Originally Posted by cimplexsound

My brakes started acting up on my Tioga recently. My mechanic and I have been through thick and thin trying to figure out what could possibly be wrong. After I drive it about a mile the brakes will freeze up and I can't go anywhere. First symptom I can't get it past 25 MPH, then I have a hard pedal, them I'm stuck on the side of the road with frozen brakes. We're already $600.00 into repairs. We both think the master cylinder is shot. But he is checking the brake booster just to make sure that is not a problem too. My mechanic had to come out and rescue me twice yesterday. By loosening the mounting screws on the master cylinder to relive pressure on the system we were able to get it moving again long enough to get it back to the shop for repair. I never had a problem last winter. This big beast got me to Arizona and back without any trouble. And she runs great. It's just the brake problems. Napa Auto has the parts if that is indeed the problem. I am anxious to get out of here before the snow flys. So what do you think?? Everything else is in perfect working order. What could be wrong???

Well,
As has been stated, you don't say what make/model/engine/year/type of coach you have so, some of that info might make a difference in what kind of answers you may receive. Second, from your description of a temp remedy of the problem, you (and your mechanic) loosen the nuts from the brake power brake booster to the master cylinder which, apparently allows for the piston in the master cylinder to RETURN to the AT REST POSITION, which, allows for the fluid in the system to relieve its own pressure by re-entering the storage chambers in the master cylinder.

You see, in a drum brake operation, hydraulic pressure in each wheel cylinder is applied to the shoes, to push them outward, towards the drum, to apply brakes. Then, when the foot is removed from the pedal, there are some fairly strong "return springs" that will pull the brake shoes, back to their home position and, along with that, the fluid used will travel back to the master cylinder and, all is well since the shoes are no longer against the drums.

In a disc brake operation, there are no RETURN springs. Only atmospheric pressure is existent and no pressure in the system is there, when the pedal is released due to the large spring in the power brake unit, that allows for the piston to do a full return.

Now, to me, that's kind-of pointing at the power brake booster. In a power brake booster, vacuum used to assist the pedal in applying pressure to the piston in the master cylinder. But, there is also a large return spring inside that chamber that pushes the diaphragm back towards the driver, so that the piston in the master cylinder can return to the "at rest position" when the pedal is released. And, when that happens, like stated, that will open up the ports in the master cylinder to allow for the brake fluid that was used for pressure, to return to the holding chambers.

And that means no residual pressure is left in the system. Well, it appears that you're system IS HOLDING PRESSURE! And, that when you back off the retaining nuts that retain the master cylinder to the booster, the piston is allowed to self return and, your pressure is now relieved and, your brakes are not held in the "on" position.

So, with all that being said, if you've done this "back off the nuts thing" a few times and it cures the problem, EVERY TIME, then I'd take a look at that brake booster and see what, if anything can be wrong, with it's internal free travel, that would limit the master cylinder to return to it's home position. Just a thought here. Let us know what you find out.
Scott

As a side note, while my theory might be valid, it could be just the master cylinder causing the issue. But, if it was, then releasing the retaining nuts that retain the master cylinder to the booster, should have no effect. It would still be holding pressure. Again, just some thoughts
Scott

Ziggy8321 · 12-12-2015, 12:00 PM

Quote:

Originally Posted by Jim_HiTek

I've also heard of this type of symptom and in that case it was older hose that the inner layer would age out and a flap of rubber would drop down into the hose after the RV'er had applied brakes a few times. It was temp sensitive so it appeared random. It would block the return flow of fluid so the brakes would stay applied.

Owners would usually just replace all the brake hose throughout the RV because there were bad batches of hoses produced over the years.

It would help if you told us what year, style, and model of RV you have when you post about a problem. Saves shooting in the dark somewhat.

X2

I have had friends experience this. Both Ford and Chevy with Chevy being more often than Ford with the hose deterioration.

twinboat · 12-12-2015, 12:10 PM

What Fire Up said makes the most sense in your case.

If you check the temp of your brakes, they all should be hot, since they are applied.

Vacuum brake boaters ( old stuff ) build vacuum from the engine in the large double sided chamber with a diaphram in the center.

In normal mode there is the same amount of vacuum on both sides of the chamber.

When you depress the pedal you release the vacuum from the firewall side in proportion to how fast or hard you step in the pedal. The vacuum on the master cylinder side pulls the diaphram towards the master cylinder, boosting your effort at appling the brakes.

If the centering valve leaks the brakes will self apply. If the belcrank links behind the booster bind the booster senses pedal pressure.

I am assuming you have an old Dodge MH with vacuum booster brakes.

If you have hydrolic assist brakes working off the steering pump, it's works the same way but using pressure not vacuum.

cimplexsound · 12-12-2015, 02:59 PM

I'm fully aware of how the brake system works. Done a lot of work on automobiles of all kinds. We have determined a bad vacuum booster. The pedal won't go to the floor when the motor is started. Then of course the brakes lock up after several depressings of the brake pedal. So the check valve is gone. The only thing left to do is replace the booster