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MG MGB Technical - Soft pedal after brake refurb

Hello all,

The following have been replaced on my '75 GT:- Calipers, pads, front hoses, rear wheel cylinders, shoes, hose, and a new master cylinder fitted. The problem is that despite countless bleedings, the brake pedal remains very soft. When the rear hose is clamped off, pedal is soft. When one front hose is clamped off, pedal is soft. When both front hoses are clamped off, pedal is solid. Possible problem between front hoses and calipers?
My very frustrated mechanic of about 40 years experience has run out of ideas. Would be very grateful for your suggestions......much money and time spent refurbishing brakes and the car is not comfortable to drive......your help is needed, please!

Many thanks, Clark.
Clark

Clark,
Are you saying that the pedal is soft unless BOTH front hoses are clamped? What brake fluid are you using? If you converted to silicone, it can take a significant amount of time to get the bubbles out because they like to just hang in the fluid in suspension rather than drifting to the top. It also sounds like you might be having trouble getting fluid to the outside piston of the calipers. Push the pistons back in all the way on both sides of both calipers and bleed again. Eezi-Bleed is a gem beyond price in these conditions.
David "Bleeding fool" Lieb
David Lieb

Hello David,

Thanks for your quick reply. Thats right, pedal firms up only when both front hoses are clamped. Recon brake fluid is DOT 4, not silicone. Thanks for your suggestion.

Clark.
Clark

My best recommendation in that case is to loosen both bleeders, force all four pistons all the way back in their bores (as if you were going to change the brake pads), then bleed with a pressure device like the Eezi-Bleed, followed by a normal bleeding.
David "I guess one mush make blood sacrifices to Lockheed as well as Lucas" Lieb
David Lieb

Are you sure you got the right side caliper bolted to the right side of the car? Same with the left?
Bleed screw must be at the top. Check with section M in your Bentley manual.
Kimberly

I overhauled the front brakes this weekend too - new seals, pistons, hoses and pads. Before the job my pedal was really firm, but now it's not. I suspect there are two factors involved - one is that the reason for the overhaul was the calipers were slow to release and so probably there was less actual movement in them than there is now (that could tie in with yours feeling hard with both front hoses clamped), the other is that new pads will always feel springy (rather than spongy) until they've bedded in. So long as your brakes are working well I suggest you leave them for 200 miles or so and then see how they feel.
Miles Banister

Kimberly. Good thought. I did not consider this when thinking about the problem.

Clark. Kimberly, David, and I are all used to the North American specification cars. The NA spec car, of your year, had a brake booster (servo) between the pedal box and the master cylinder. The master cylinder is a dual system with the rear reservoir operating the front brakes and the front reservoir operating the rear brakes. (Just went out to the garage to verify this in my memory.) However, I do not know if your car is a NA specification or a Home Market car and I am not sure what brake system was used on the Home Market cars of that year.

That being said, Kimberly has an excellent suggestion to check to make sure the calipers are installed correctly. Do that first, as it is fairly easy to to.

Next, you have isolated the fault is being in the front brake system which includes the rear reservoir of the master cylinder, two hard lines coming off the rear reservoir and going down to a bracket on the chassis near the wheels, two flex hoses, and two calipers. Any portion of this system may have a problem which would cause a soft pedal on braking.

Master cylinder. These have been known to be bad from the factory. You need to determine that there is fluid being pumped from the master cylinder reservoir to the calipers. I am assuming that you bled the brakes by having an assistant press the brake pedal to the floor while you opened the bleed nipple. If so, and you got fresh brake fluid out, you have verified that the master cylinder is pumping fluid from that reservoir. If you did a reverse bleeding procedure, check to ensure the master cylinder is pumping fluid out of the rear reservoir.

Use a line wrench to check the tightness of the brake line fittings into the master cylinder. Before doing this, feel for any brake fluid that may be around the openings. If the fittings are night tight, fluid can leak out and air can get into the system.

Do a visual inspection of the hard lines, looking for any brake fluid which may be leaking out of a cracked line.

Verify the tightness of the flex line to the hard line.

Verify the tightness of the flex line to the caliper, ensuring that the copper washer is between the caliper and the flex line--it provides your seal at that point.

Check for leakage around the joint line between the caliper halves.

Check for leakage around the piston seals.

A leak in any of these areas can allow air to get into the system and cause a soft pedal.

If you find that the master cylinder is pumping fluid out of the rear reservoir, and that all of the connections are tight and there are no cracked hard lines, it is time to follow David's advise and push the pistons back into the caliper bores, then, begin the bleeding process again. Be aware that forcing the pistons back into the calipers will displace fluid back into the reservoir. Use some form of suction device to remove about half of the fluid from the reservoir before pushing the first piston back, then check to make sure it will not over flow before you push each other piston back.

Let us know what you find out.

Les
Les Bengtson

Check the bleeder orientation first as Kimberley suggested. If bleeders are correct this article may help.
http://www.stoptech.com/tech_info/wp_bedintheory.shtml
It's normal for new brakes to be somewhat soft until the new parts are bedded in. How soft is determined by how well the parts are matched and the condition of the rotors. A new rotor will usually cause soft brakes due to it's rough finish. You didn't say the rotors were replaced or resurfaced? If you are using old rotors they may be glazed, have small wear grooves or a ridge near the outer diameter of the rotor. If the pads don't match the old wear surface or the brake pads extend over the outer ridge the pedal will be soft until the pads are bedded to the rotors.

Clifton
Clifton Gordon

With both my cars I always have to bleed the brakes in two stages. The first is with either pedal or EeziBleed, but that always leaves it with a soft and long pedal that will pump up, but goes long and soft again after a few seconds. The second stage is having someone press down hard on the pedal while I rapidly open and close each caliper nipple in turn. That always gets an extra lump of air out whereas even continuous (i.e. EeziBleed) low pressure bleeding doesn't and they are fine after that. Whether it just needs the very high pressure to blast the air down the pipes and out of the calipers, or whether it needs the high pressure *and* flow to consolidate what may be microscopic air bubbles all along new pipes and hoses because the fluid hasn't fully wetted the surface I don't know.
Paul Hunt 2

Thanks all, very helpful,

Kimberly, I will double check, but I *think* calipers are on ok. Did not fit them myself,helped with the bleeding, but will check again.

Miles, I take your point about new pads, etc. feeling springy, but the feel is downright spongey, and brakes are not working well......almost certain they won't improve with use.

Les, car does have servo but not a dual system. Have verified that the master cylinder is indeed pumping through calipers and that all lines and connections are tight and with no leaks.

Paul, thats exactly the feel of the brakes. Perhaps they do need a good blast.

Thanks for all your suggestions, will let you know when problem is (hopefully) cleared up.

Clark.

Clark

Clifton, the calipers are not new but are rebuilt *exchange* units. Thanks for the link.

Clark.
Clark

Someone else wasn't able to bleed his brakes.
The problem and solution are in the following thread.
http://www.mgexperience.net/phorum/read.php?1,759447
Kimberly

Thanks for that, Kimberly, we are having another shot at it tomorrow.

Clark.
Clark

Hi. the only thing I can add is to run the engine when bleeding the brakes.
William Murphy

I have heard of putting a board between the seat and the brake pedal tightly overnight. I lived with mine being soft for a year or so, and one day "voila". It now is firm and very little travel. The bubble must have worked its way out. Not that I recommend being as stupid/lazy as me..
Tom

Thanks to all for your time and helpful suggestions. Went with Paul's method at the weekend and now have a much firmer pedal. Will run for a few hundred miles then probably repeat the procedure.

Thanks again,

Clark.
Clark

This thread was discussed between 23/03/2008 and 01/04/2008

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