OT -- disc brake problems

[Coyote]

There is a disc brake on my son's MG that refuses to relax after being applied. The wheel gets so hot you could fry eggs on it. Like all disc brake systems -- it's bone simple. The caliper is a non floating opposed piston type (dual piston - one on each side of the rotor).
What are the posible things that would not let the brake not let go? The MG is his only transportation and is currently parked because of the problem. We have pulled the caliper but can't find a thing wrong with it. While the caliper was off, I pumped about a cup and a half of clean brake fluid theough the line. Put everything back together but it changed nothing. Apply the brakes once and that wheel (front passenger) is locked.

[rbond]

I assume both pistons came out easily and did not have a rust band that might prevent them from retracting? If so, take the brake 'divider' (I know that is not the correct name) apart, it should have a switch on it when the brakes lose fluid it closes off the leaking side and turns on the 'brake' light. It is designed to give you at least one back and one front brake to help you stop somewhat safely. Next would be the master cylinder. Take it apart to clean it, it might not be letting the fluid return to the reservoir thus keeping the pressure on. I had a 1963 VW beetle do this on all four wheels. It was a simpler systems so they all worked or all failed. Could not get the correct size master cylinder kit, so I just cleaned everything very good. Did the trick. There is a small possibility that the rubber line to that wheel has a tear inside that closes on fluid trying to return, thus blocking it and holding the pressure on. Replace all the rubber hoses if this is the case. The Brit's hydraulics are fairly straight forward, it will still take some time trouble shooting each component. Good luck!

[Zook-e]

Sounds to me like your front brake line insides have imploded. They are allowing the fluid to move to the caliper but not return after letup. I had the same problem on my 1958 Buick 66R. It happens more often than you may think.

[rngdng]

Sounds to me like your front brake line insides have imploded. They are allowing the fluid to move to the caliper but not return after letup. I had the same problem on my 1958 Buick 66R. It happens more often than you may think.

I've also seen this happen. It's certainly worth checking.



Lane

[djfisher22]

Sounds to me like your front brake line insides have imploded. They are allowing the fluid to move to the caliper but not return after letup. I had the same problem on my 1958 Buick 66R. It happens more often than you may think.

That's where I'd put my money.
Just had to replace rubber caliper hoses on a Cougar I just sold.The R.S. was locking in the rain. Not so odd an occurence.Change em both.

[Buffalo-guy]

When you had the calipers apart, did you remove the seal rings and clean the groove out? My rotary did as you described, and cleaning out the crystalized crud from behind the seal did the trick, and allowed the piston to move freely. Just a thought. Cheers.
Fred

[Craig380]

my old MGB GTV8 had sticking front calipers, I once had a pad fire because of it

I too tried the trick of pumping the pistons in and out, and cleaning up the visible bits. Didn't work.

I don't know how the spares situation is in the US, but in the UK a refurbed caliper is about 70 bucks and delivered next working day, fitted easily, solved the problem.

Here, it's actually cheaper to get a complete refurbed caliper than it is to buy a seal / piston kit.

[bikegeezer]

There is a disc brake on my son's MG that refuses to relax after being applied. The wheel gets so hot you could fry eggs on it. Like all disc brake systems -- it's bone simple. The caliper is a non floating opposed piston type (dual piston - one on each side of the rotor).
What are the posible things that would not let the brake not let go? The MG is his only transportation and is currently parked because of the problem. We have pulled the caliper but can't find a thing wrong with it. While the caliper was off, I pumped about a cup and a half of clean brake fluid theough the line. Put everything back together but it changed nothing. Apply the brakes once and that wheel (front passenger) is locked.

Coyote,
In the bottom of the master cylinder reservoir, you'll find two small holes. One is about the diameter of a human hair and can plug up easily. When that happens, pressure in the lines can't bleed off back into the reservoir. If this is your problem, the stuck brakes will release when you open the bleed screw on the caliper. Fixing this requires overhaul of the master cylinder - don't poke anything metal into that hole. Spray carb cleaner and compressed air only.

Stu

[krwalsh]

Having had this same exact issue on my Austin Healey Sprite (MG Midget), I will bet you that it is not the caliper. As said above, your problem is in the master cylinder. It is one of two things: The pressure relief port is blocked. It is a tiny little hole, and the smallest amount of crud will plug it. Depending on the year, the master cylinder is separated into two pistons, in an attempt to separate the front from the rear. So, you have to fully disassemble the master to make sure there is no rust, crud, or a shredded seal. Finally, if the stop screw for the pedal is adjusted too far, the pedal will not return all the way, which is what lets this relief hole open. From there it is a vicious cycle of the caliper heating up, which expands the fluid, which binds the caliper more, which puts more heat in....

So, once you've verified the caliper is clean, I'd go for the master.

[Barry S.]

I had a GMC pickup do that, the rubber brake line was bad. I replaced the caliper and it still did it, somebody told me it was the rubber brake line, sure enough it was.

[Suzsmokeyallan]

If the bleed back hole in the master cylinder was clogged it would cause all the brakes to stick, but only one is so you can sort of discount that area for now.
I'd suspect its from the splitter block to the caliper itself, if you do not know how old the hoses are, replace them for safety and because you should anyways.
I'd go for the imploded hose theory for now plus its the cheapest solution.

[bikegeezer]

If the bleed back hole in the master cylinder was clogged it would cause all the brakes to stick, but only one is so you can sort of discount that area for now.

He said it's an MG, but I was still thinking motorcycle

Stu