Rear brake very hot

[wwheeler]

I am having trouble with my right rear brake getting hot.

Some history first. I discovered one of my front wheels did not spin easily when off the ground. Bearings were adjusted correctly and turns out the pistons in the caliper were tight. I "exercised" them to loosen them up which worked well. I then decided to do a preventative check on the others.

One of the things I found was the outer piston on the right rear caliper was totally seized and it's dust cover was deteriorated. The outer pad was very thin but the inner pad (piston still free) was fairly thick. I concluded that the dust seal had deteriorated and allowed debris to seize the piston. I replaced the caliper and pads with new MB pieces and thought my problems would be over. When replacing the caliper, I noticed that brake fluid would continuously drip from the brake line (3 drops /sec) unless capped off.

After a 20 minute test drive with the new caliper, the caliper was very hot to the touch and the other three were cool. There was even a slight smell of bunt pad coming from that wheel. I did some hard stops in forward and reverse hoping that would loosen the pads. After the next test drive, same thing.

I next removed the wheel and brake pads. The pads were tight on the rotor as the rotor would not budge. While removing the pads, I was able to move the pistons back in their bores, but they seemed to push back by themselves. My guess is that the caliper got very hot with the PO and burnt the dust seal and seized the piston and has been idle ever since.

What would cause just this one caliper to engage but not the others? Could it be the brake valve in the rear?

Thank you for your help

[hkollan]

Did you replace the flexible brake hoses? With age they get swollen on the inside and prevent the brake fluid from returning freely and could prevent the caliper from disengaging. They are cheap and it's a good idea to replace all four if they are a few years old and you are experiencing this kind of problem.

[ja17]

Hello Wallace,

Yes,  my guess would be with Hans on the brake hose.  You may also want to make sure the emergency brake is releasing on that side. Most likely the hose.

[wwheeler]

Thanks for the ideas. I had all of the hoses replaced and the fluid flushed 3 years ago when I bought the car. So I have been hesitant to blame this problem on the hoses. When I remove the pads, the rotor is loose, so I don't think the emergency brake is too tight. All of the major brake components are original if that helps.

I can push the pistons back in their bores by hand using the pads. The pistons go in fairly easy but push back out in about a second. The pads aren't so tight that they lock up the rotor, but they are snug on the rotor. Since the caliper is new, I thought that it still may have air trapped. The way the pistons moved back almost seemed like trapped air. I bled it again two different ways. Using a vacuum pump and then the two man method with the brake pedal. I got some air with the vacuum, but absolutely none with the two man method. So I think all of the air is gone but the pistons still spring back. 

I remember that when I exercised the front calipers, the pistons stayed in the bore and didn't push back on their own. That seems normal. It almost seems like there are springs pushing the pistons back out on this rear caliper.

When you disconnect the brake line to the calipers, is it normal for the fluid to continuously drip? The line to the rear caliper did this until I put a rubber plug in it and found a new caliper. It wasn't any pressure, but just a constant drip every second. I don't know if this may be related to my problem.

This is a frustrating problem. Thanks for your help!

   

[badali]

Are the bleeders in the top hole and brake line in the bottom hole?  Air will be trapped if not.   Fluid will drip by gravity when system is open.

[wwheeler]

Yes, the new caliper has the bleeder on top. I did check that after reading a thread about that very subject here recently, thank you.

Looking back at my first post, I am wondering if my first caliper suffered from the same fate. The outer pad was very thin while the inner pad was at least 3 times thinker. Why was the outer pad so thin? Maybe it was the same force pushing the pistons out then as it is now. Could it be that the brake got hot enough for so long from the pads contact on the rotor, that the dust seal hardened and the piston froze? Strange that it is only this one side. The fronts are definitely OK.

I already checked the rear caliper on the other side but will do it again to make sure it is OK and normal.

Any thoughts are greatly appreciated.

[ctaylor738]

After my recent brake debacle, I am in no position to advise, but I would definitely replace the hose because it's cheap.  While it's off, suggest you make sure the metal line is clear.  The final possible culprit is the pressure regulating valve.  I do recall a post on its causing a lack of braking at the rear wheels.

[graphic66]

You should also "fit" the brake pads to the caliper. They almost always need to be filed down a little on the metal edges of the backing plate so they move in the caliper easily. This can also help reduce brake squeel. Maybe not your problem, but it is not a bad idea to do this. I also put a very small amount of never sieze on the edges of the metal backing plates where the ride on the caliper. Does opening the bleeder release the brake? If so the hose or valve could be the problem, if not it is most likely your caliper.

[wwheeler]

I think I have solved the problem but won't know until I take test drive this weekend.

I loosened the bleeder and pushed in on the pistons until they bottomed out. Some fluid came out in the bleeder tube I was using as expected. I closed the bleeder, but the pistons sprung back as they had done before. I kept trying it and noticed each time I did it, it took longer for the pistons to spring back. Encouraged, I did the same procedure 10 more times at which time the pistons stopped springing back!

I never saw very much air come out in the bleed tube, but am sure there must have been an air bubbled lodged near the  pistons. Do you typically need to bench bleed a caliper like you do a master cylinder?

Anyway, pistons stay put when I push them back. More importantly after I apply the brake, the rotor is free to turn and the pads just touch rotor instead of binding. 

Thank you for all of your help and suggestions.     

[jacovdw]

Wallace,

From your description it sounds more like the new seal around the brake piston was a bit too tight and therefore not allowing the piston to return to its rest position.

With you exercising the piston a couple of times, it caused the seal to loosen some of the grip on the piston and hence the easy return.

I have seen this happen a few times with rebuilt calipers and usually a bit of exercise relaxes the seals a bit.

Typically you do not need to bench bleed the calipers as long as you install them the right way - bleeder always at the top.

[ejboyd5]

It does appear that hysteresis may have contributed to the dragging brake.  Good luck with the repair.

[wwheeler]

An interesting thought. Maybe would be a good idea to exercise the pistons a few times before final installation. The front caliper on that side was also replaced and I didn't have any problems with that one. That caliper had a broken bleeder with a broken easy-out from the PO. No way to get that out!

Thanks for the help.
 

[graphic66]

Sometimes you can weld a nut over the broken bleeder screw by welding the open center of the nut to the broken bleeder and turn the nut and the bleeder comes out. Make sure you don't have rubber seals near the heat.

[wwheeler]

This bleeder unfortunately broke off flush with the casting!!!! On the original caliper castings, the bleeder is in a 3/8" cavity. So this was way down in the cavity. If I ever have another (I hope I don't), I'll remember you clever trick.

[dseretakis]

Ah the famous broken bleeder! This has happened to me before necessitating the purchase of a new caliper.  I'm about to replace a rear rotor and pads on my pagoda and would like to also replace the rubber brake hose. The bleeder screw is not budging so I'm kind of hesitating to replace the hose. I might apply some heat to the bleeder and see what happens.

[wwheeler]

Others might chime in who have done this sort of thing. I would use PB blaster several times first before applying heat. I once had to loosen the coolant elbow in the head. It wouldn't budge until a week's worth of PB blaster applications. Be patient. It will work eventually.

The old seals in the caliper as well as the brake fluid may not like the heat. I don't think the PB blaster (or any penetrate) would cause any damage to the brake system.

[wwheeler]

The dragging rear brake and high temperature still persists but at this point, I don't know if it IS a problem.

In this process, I have learned something about the rear brakes. The rear pistons (on my 111 anyway) have have a spring loaded compensator that rides on a pin in the caliper bore. I found info about it in the BBB. It is to compensate for the rear rotor runout. The BBB mentions that the rear pistons require more air pressure to pop them out and that they are harder to move in their bores. The springs would also account for the pistons "springing" back when I pushed on them as stated earlier.

I did replace the brake hose and it wasn't a problem. I rebuilt the rear caliper anyway with new seals and dust seals. The pistons do seem to be a bit looser now than when I first received the new caliper. But after another test drive, the right rear brake was hotter (not as much so) than the other three. I removed the tire and spun the rotor and it was not as tight as before but more so than the fronts.

Is anybody familiar with this brake compensator feature? Does it just take some time to burn off some off the new pad before it loosens up? Or will it always be tighter and run hotter?
Anybody have experience with this?

Thanks for the help!

[ja17]

Hello,

Keeping those little rubber caps on the end of the brake bleeders keeps droplets of road water from entering the bleeder and rusting the bleeder threads stuck.

As last resort tap on the end of the bleeder with a small hammer to loosen rusted threads after squirting it with penetrant. Make sure you tap straight on and not from the side or you will break it off !

I also like the heat method to get bleeders unstuck.  Calipers are designed to handle a lot of heat so heating the bleeder is not an issue.

[wwheeler]

An update to this thread. I am still having problems with the right rear brake getting hot, but have some new thoughts.

I replaced the new caliper with another brand new ATE caliper to eliminate the possibility of the caliper being bad. I noticed when I installed the new caliper, you have to be very careful to have the rotor centered in the caliper housing and secure. Because the rotor "floats" on the rear axle flange until the wheel is installed, the pistons positioning themselves for the first time can move the rotor out of center. The rotor has to be secured first. That was one problem.

I also found out that the caliper ONLY gets hotter than others when driving at high speeds for 20-30 min. or longer. The caliper will NOT get hotter just driving and braking on the city streets. The rotor itself is not that hot. My new theory is that the right axle bearing may be worn and causing the rotor to bind into one pad more than the other with load from the wheel. Because of the piston spring compensator's, the piston will not retract as easily. When I "centered" the rotor and bled the brakes, it was with the wheel off the ground. If the bearing is worn, a load would cause the rotor to shift. My axle has 140,000 miles on it.

The other piece to this puzzle is the condition of the original caliper that had the outer piston extended and frozen with the dust seal cracked (see first post). That outer pad was very thin while the inner pad was thick. I wonder if a worn bearing could cause the rotor to shift causing a high load on one pad? This therefore would generate high heat and degrade the seal and finally seizing the piston. 

I could sure use some advice with this one!

[Naj ✝︎]

Just a thought, Wallace.

Did you check the e-brake shoes while you had the caliper off?

On one of the UK Club members cars, the shoe linings had parted from the shoes and caused all sorts of noises and problems.

Naj

[wwheeler]

Thanks Naj,

I did check the brake shoes and they looked OK. Without the pads in the caliper, the rotor can spin freely. When I apply the hand brake, the shoes lock the rotor. The rotor spins freely again when I release the brake. Strange why that one side is so hot. At its worst so far, I would guess that the RR caliper is around 170-180*F to the touch, while the other calipers are 110*F or so. Quite a difference. I can't hold my hand on the caliper for more than a half a second before it gets too hot.

[thelews]

Is the rotor hot too?

[wwheeler]

I check the calipers right after a drive so the wheel is on and the rotor is hard to access. But the wheel rim, rotor dust shield, axle housing and anything else I can feel are NOT hot. In fact, these parts are the same temperature as on the other side.

I can always tell when the caliper becomes hot during a drive, because the brake on that side will let out a slight squeak only just as I apply the brake. As I mentioned earlier, I replaced the caliper on the left rear side and have no issues. So at least I did that one correctly! 

Keep the ideas coming.

[jacovdw]

... If the bearing is worn, a load would cause the rotor to shift. My axle has 140,000 miles on it.

Hello Wallace,

Interesting problem indeed and I'm following this with interest.

One way to verify if the bearings are suspect/bad is to measure the amount of play in the rear hub with a dial gauge.
The process is similar to the one used to check the bearing play on the front hubs.

The other piece to this puzzle is the condition of the original caliper that had the outer piston extended and frozen with the dust seal cracked (see first post). That outer pad was very thin while the inner pad was thick. I wonder if a worn bearing could cause the rotor to shift causing a high load on one pad? This therefore would generate high heat and degrade the seal and finally seizing the piston. 

It is possible...

Generally speaking, it is usually not the heat that seizes the piston, but rather the crap that enters when the dust seal is cooked/cracked. Moisture finds its way in the space under the dust seal and rust quickly develops in the space between the dust seal and the piston seal in the bore.

Also important is that the mating surface between the brake rotor and the hub is absolutely clean and free of corrosion/grime to ensure a perfect fit.

[Naj ✝︎]

Anyone know if there is a rule (of thumb ?) when the rear axle bearings should be repacked with grease?

Naj