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MG MGB Technical - brake bleeding

My 1967 MGB poured smoke from right front wheel well after recent short drive. Car is "summer only" "nice weather" driver only. Upon inspection, right front caliper stuck. Opted to replace both front calipers with new ones. Believe I correctly bled system afterwards (got clear stream from bleed screw) but brakes now soft/spongy on first push, pedal depresses about 1 1/2 inches. Second application pedal firm, depresses about 3/4 inch. Is this a bleed problem ??
charles updegraph

Charles, I had exactly the same problem when I replaced my front calipers and servo (brake booster). Have a look at this link from Paul Hunt's website:

http://www.mgb-stuff.org.uk/braketext.htm

Text copied below

if the fronts have been dismantled and air has got into the system additional steps are needed, as after using the gunsons alone I get a 'long' pedal. This can be 'pumped up' but after only a few seconds that effect is lost and the next application results in a long pedal again, which can be a bit disconcerting! I get my assistant to press down hard on the pedal while I rapidly open and shut each caliper bleed nipple in turn. The much higher pressures, and hence flow when the bleed nipples are opened, seems to blast any remaining air out of the pipes. Thereafter the brakes have always been as expected. Note that on twin-circuit brake systems this may well cause the shuttle valve to move and light the brake warning lamp. If it is still lit after doing both sides then on the side opposite to the one you did last, get the assistant to apply medium pressure to the pedal, open the bleed nipple just a fraction letting a tiny dribble of fluid out, get the assistant to shout as soon as the warning light goes out, and rapidly close the nipple. If it doesn't go out try doing it on the side you bled last.

I've used this method very successfully a few times in getting an assistant to push down quickly and hard on the brake pedal and blasting the air out. This was for curing front brake squeal, but that's another story.... :-)



Andy

Andy Robinson

Makes sense. I"ll try it. Thanks
charles updegraph

First, make certain that your calipers are on the proper sides. It is very easy mix them up and when you do, bleeding them is next to impossible. The bleeder screw should be at the highest point of the caliper to purge all air from the system. A common mistake when replacing both calipers. RAY
rjm RAY

A problem that's been cropping up recently is calipers that are not setting the pads up right. What is happening is the pads go all the way to the disk when you press and bleed them (no air in system showing) but then the calipers allow the pads to retract all the way back so they're loose in the caliper. It makes it continually feel like the brakes are out of adjustment (if it were drum brakes) and leads to the double pumping requirement.
The way to fix it, is to take one pad out at a time and push the piston about half way out, then manually push it back in, put the pad back and move onto the other side, or other caliper.
Maybe the caliper has been sat on a shelf too long or something making the dust seals too 'sticky'
Roadwarrior

In case anyone has ever wondered why 'double-pumping' does what it does, there is a one-way restrictor in the master cylinder. This fully opens to allow maximum fluid flow when the pedal is operated, but closes when the pedal is released to leaves only a very narrow channel for fluid to get back. http://www.mgb-stuff.org.uk/mcvalve.htm
Paul Hunt

Iíve only ever used the 2 person method and have never had an issue, albeit Iíve never used it on a vehicle with a shuttle valve. I think my distrust of the one person method is nothing to do with its abilities or lack of, but as a young lad I watched my father bleed the brakes on my motherís Ford Zodiac. The device he was using was a bit like a spring loaded bicycle pump that was filled with brake fluid, it had a short flexible pipe on the end that had a cap on it that screwed to the master cylinder. The idea was to screw the cap on the master cylinder and release the tension on the spring. For whatever reason the cap wasnít properly attached and come off spraying brake fluid everywhere.

It was also on the same car that I managed to get the battery to explode. The battery was on charge, with a very large charger, and I thought it would be fun to run a brazing rod across the terminals. It blew all the plugs out, one of which cut me above the eye, and a small section of the battery top. We found all but one of the plugs. I now have a great respect for health and safety.
R.A Davis

Update: Using a combination of the ideas above, I believe I now have a well-bled system. A new issue has arisen, namely that while testing the brakes (on my driveway) the left rear (drum) brake locked up (left rubber.) I assume it is time to reset the "clicker" adjustment on the rear. For what it is worth, during the front caliper bleeding, the master cylinder reservoir was never allowed to empty, so I don't think this relates to my bleeding odyssey. Am I missing anything ??
charles updegraph

Charles, There is only one flexible for the rear drums. Mounted on the right side of the axle. If one is sticking on it could be for the usual reason which is a seized cylinder.
Allan Reeling

Or your rear axle seal may be leaking 90W oil onto your brake shoes. This will cause the shoes to lock up the brake drum. Just the opposite of what you would expect. RAY
rjm RAY

"I assume it is time to reset the "clicker" adjustment on the rear."

They are not self-adjusting so they should only ever get slacker during use - although having said that MGB rear brakes wear slower than a glacier sliding down a mountain. However as well as axle oil causing them to grab I've also had a problem whereby one side suddenly started locking up each time I tried to reverse, when I hadn't altered anything. No leaks in that case, and the only thing that resolved it was to slacken off the adjuster by one click.

You also need to check the orientation of the shoes, there are probably half a dozen different ways of fitting them, and only one is correct. The correct way is to imagine a point on the drum passing over the shoes when the car is moving forwards. Each shoe has the friction material fitted towards one end of the shoe, leaving an 'empty' section at the other end. A point on the drum should pass over the empty section first, both shoes, both sides. There is also a choice of which holes in the shoes the springs can be fitted, and which way round the springs go, and again there is a right and a wrong way. Generally the right way leaves the springs free from obstruction and rubbing on other parts. If that weren't enough the handbrake levers can be fitted to either side, and again the wrong way can leave them fouling the shoe and the slave, not helped by various drawing showing them sometimes one way up and sometimes the other.
Paul Hunt

I have not pulled the wheels yet to check, but my sense is that the left rear functioned properly even though it "left rubber" because I really pressed down hard on the brake pedal. Following that reasoning, I thought the right rear might need a one click tightening so it would also lay a strip. These cars did not,of course, have ABS capability so locking up might be expected so long as it is even between wheels. FWIW--both wheels spin effortlessly when off the ground.
charles updegraph

Ah, I see "Left rubber"i.e., in F1 terms "flat spotted".
Locking up of the rears and not the fronts is unusual, as the fronts do 75% of the retardation and are much more efficient. If a rear is "leaving rubber" then you are dragging it and therefore it's locking long before the fronts have done their work. Less likely to be a cylinder problem as you can't put sufficient pressure in to lock a drum on it's own. Pressure proportioning on a B is down to cylinder size. More likely to be a shoe problem. The lining can detatch from the shoe and lock everything up. As said above oil (axle or DOT 4) contamination could do it, (and also detatch the lining!) also shoes fitted the wrong way round, see Paul's post.
Allan Reeling

New shoes and dismantling/refitting/adjusting differently aside, the shoe should bed in evenly to the drum and have equal pressure applied to it as the fluid pressure is the same both sides. Also the front to rear balance is designed so that it doesn't lock before the fronts in the wet, let alone the dry. So one wheel locking will be down to one of the other factors, not simply adjustment.

Roadster cylinders were reduced in size quite early on to prevent locking, the GTs then got the larger cylinders as they are heavier, but the V8s used the smaller ones perhaps because of the harder suspension. So with axle i.e. back-plate swaps it's possible to get the larger cylinders on the later roadsters, and maybe one of each, it's the position of the location roll-pin that determines it.

The other side of the coin is that I had to replace the V8 shoes recently following both wheel cylinders leaking, and the braking performance from them became abysmal. I could see the wear pattern showed loads of friction material being unused, and have had them off several times relieving the 'high' spots. They are better, but still nowhere near what they were, or compared to the roadster. My drives have been on a slight slope and I frequently roll the cars out of the garage and lean in and pull on the handbrake. Before, and with the roadster, they locked, but not now. I was concerned that it might fail the MOT but didn't, but then testers have commented in the past about how good both cars were.
Paul Hunt

I was talking to a couple of garage mechanics recently and both have had cars in, both classic and modern....ish, all with linings separated from the shoes!!!! Bad product again!!!
Allan Reeling

End of the story---I pulled both rear drums this week-end and all was normal. Adjusted the right rear slightly. Back together and the braking test on my driveway was normal. We had our first hard freeze this weekend so it is time to drape the roadster for the winter and look forward to warmer weather. Thanks to all for your help.
charles updegraph

This thread was discussed between 13/10/2015 and 19/10/2015

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