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MG MGA - Sticking brakes
|I know this subject has been discussed many times but I have ran out of ideas. I have rebuilt every last nut & bolt on my 1600 Mk. 1 race car including resleeving the master cylinder, new seals in the master cylinder and front calipers, new rigid & flexible hoses and a brand new servo.|
First time out the brakes stuck on, I released some pressure by opening a bleed nipple and everything freed up. I rang Bob West and he advised removing 25 thou from the main piston in the master cylinder which I did and they still locked up.
I then by-passed the servo and they still locked up, so I had new seals fitted to the calipers but this didn't help. Where do I look next? any ideas would be welcome as usual. I am using silicone fluid by the way.
If by opening a bleeder the brakes release themselves it indicates to me that the fluid is not returning to the master cylinder reservoir and that the wheel cylinders are fine. This can be caused by the cup not returning far enough to clear the smaller of the two holes in the master cylinder. It can also be that you do not have sufficient play on the brake pedal thereby holding the cup over the return hole.Drain all the fluid out and look down through the smaller of the two holes and you should be able to pass a piece of .030 locking wire down without restriction.Until that hole is clear you will have the same condition.
|I had a similar problem once on my coupe some years back. I too fixed it temporarily but releasing the brake cylinder bleeder. I'm not sure exactly what fixed it because I did several things at one time. I replaced the brake shoes, rebled the system and replaced the brake shoe springs. I think that the springs were the culprit. My logic, however illogical it may seem is, If the springs do not push the fluid back up to the reservoir, the M/C piston cannot return to clear the hole. I can imagined that 40 years of use the springs may have weakened and stretched. It doesn't take too much pressure to drag the shoes and create a lot of heat. I'm not sure why the added pressure in the lines didn't push the piston back by itself. In any case, when all was done, the problem went away.|
|Hi Maurice. Check that the brake master cylinder pushrod is not adjusted too tightly. The pushrod must have some clearance otherwise the brakes can lock on. The brake pedal should have one half inch of free play before the pedal gets hard when it is pressed. If the brake pedal had no play, then the pushrod likely needs adjustment. Cheers, Glenn|
|I had this exact same problem on my BMW 1800/02 ti when I was a kid. I was told it was an air leak in the power brake booster vacuum diaphragm. Being a poor kid I just got some T-fittings and plumbed around the power brake booster completely. When you say you bypassed it, did you do the same thing, or just disconnect the vacuum connector?|
Bob West's tip about taking 25 thou off the piston should have allowed clearance of the return holes that the others have referred too. It's a mod a did to my brake piston.
Thinking out of the box and probably a stupid question: I presume you shortened the master cylinder brake piston and not the clutch piston!
My reference book also says you can get the piston to travel further back by increasing the diameter of the hole in the master cylinder front plate cork gasket to allow the piston to move further back and/or by making a thicker gasket with the same enlarged hole.
|MGA's are notorious for this, silicone fluid compounds the problem. The rubbers, especially the older type as opposed to the newer type which are shinier and look more like plastic, swell and cover the return ports. When I was using silicone fluid I took far more than 25 thou (probably about double that) off the front end of the piston and the master cylinders still work perfectly even though I have not used silicone brake fluid for many years (apart from the swelling problem the lining in one of the fairly new front brake hoses turned itself into a one-way valve and that was the last straw).|
You will fire up another silicone fluid argument! Plenty of us swear by the stuff. I have used it quite happily for the last 9 years. Others have used it a lot longer.
|Thanks for the input - I forgot to mention that I do have clearance on the pushrod.|
The comments throw up two points - firstly I didn't know there are two holes in the reservoir - I'll have to take the top off to check this, and secondly, I was told to take 25 thou off the back of the pushrod [the part with the ring of small holes around the circumference] - is this correct or not?
Are you sure about your terminology? You say 'pushrod' yet you describe the piston.
|Yes, sorry, I meant the piston.|
|THE ABOVE COMMENT about silicon is dead wrong.|
Sorry to be blunt.
Silicon does NOT swell brake parts. This is one of the Disadvantages of using silicon. The regular fluid softens and swells the seals and allows them to seal against a less than perfect bore. It is also one of the advantages of silicon as it does not soften the seals and they last much longer.
In conclusion silicon is more difficult to bleed (aerates easily) and leaks through smaller imperfections but it won't absorb water, makes rubber parts last longer and most importantly, won't eat paint. If you have problems with silicon fluid you must find the problem by looking in a mirror.
I have had 7 MGAs and only the first one used regular fluid. I have never had to mill the front of any piston, though some have. The most common cause of problems is improper installation of parts followed by improperly made parts. The original parts can be modified to make up for poorly made seals but are not the "cause" of the problem.
Careful installation of good parts on a smooth bore, making sure the vent hole is open, And leaving adequate free play always works for me.
There are 2 holes in each bore. The front ones are quite large and are never the problem. The small holes look to be about 1/8 of an inch but are not drilled all the way through. The hole drilled all the way through is only the size of a needle. If this hole is plugged the pressure won't release. These are the holes that every one is talking about getting "uncovered" when the piston is returned. Run a needle through and use a light to make sure they are open. It is also a good idea to slightly stretch the return springs inside the M/C while installing them.
|R J Brown|
|Silicone fluid, all new parts, rear brakes kept binding. Didn't think about reducing the length of the piston or shimming the front. My problem appeared to be replacement pushrods were too long. I ground them down, rounded then ends a bit and off I went. Three years and lots and lots of fast mountaing driving later and not one problem.|
|It is the piston that requires modification, not the pushrod (which is adjustable!). The grinding takes place at the front of the piston, allowing it to come to rest slightly forward of the original position. All that I would say about silicone fluid is that it used to be widely used here in Oz (particularly amongst the MG fraternity, given master cylinder location -- and I was very much an advocate) but since the advent of stainless steel sleeving of cylinders (which are virtually unaffected by moisture) it is hardly used, if at all. I have owned a succession of MGA's, including the two that I still have that I acquired and restored in the 1970's. I made the qualification in my previous comment between the older type rubbers and the newer type. I cannot comment as to their composition/s but there is absolutely no question that the "older" type were subject to swelling when silicone fluid was used and this, together with the improved sleeving, were the major reasons for the discontinuation of its use.|
|Maurice, one way of shedding a little light on the problem is to jack up a front wheel immediately after applying the brakes and see the rate at which the fluid pressure reduces. If it does not reduce at all then there is a permanent pressure lock in the system and I would look at the pressure relief valve in the mc. Its job is to keep a small amount of pressure in the brake lines to keep the caliper pistons from withdrawing too much. I'm assuming here that you are able to push a piece of small fuse wire through pressure release hole and you have not got a problem with clearance.|
|J H Cole|
|I've just had a similar problem in a B anmd it appeared that it was the remote servo that was sticking on. I too had used Silicon fluid and after two servo rebuilds without success the only thing that I could think of was that it was the reduced lubricity of the silicon fluid that was causing the problem. Changed the fluid and the problem was immediately solved.|
|Thanks for the further comments. I looked closely at the 'holes' in the reservoir when I reassembled it believing there to be one for the brake side & one for the clutch & I have a sneaking suspicion the the guy who sleeved the M/C has only drilled these two out.|
I think my first step has got to be to remove the M/C to check this point. If the second holes are only the size of a needle originally, will this be critical if I drill them out oversize?
Secondly, as I would like to cover all the issues while the M/C is dismantled this time, have I done anything untoward by having 25 thou removed from the back face of the piston?
Perhaps the best course of action will be to see what's what with the holes before making any further decisions.
Any further comments would be very welcome.
|A servo on an MGA! Nice.|
|A servo on an MGA race car - I don't have one on my standard road car.|
This thread was discussed between 16/04/2007 and 21/04/2007
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