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MG Midget and Sprite Technical - Brake Problem on Mk II Midget
|Having just replaced the master cylinder on my car I now have a problem with front brakes binding. I initially thought that the caliper pistons may have seized as it had been standing a while. However on looking today I found it impossible to push the pistons back in until I slackened the bleed nipple whereupon they went back easily. On the first push of the pedal all looked OK but further pumping locked them up again. This led me to believe that the non return valve was faulty but the rears seem to be OK so this makes little sense to me. Before I remove the pedal box for the third time (first new cylinder leaked straight out of the box) has anyone got any other thoughts on possible causes? I still feel it is the valve but would expect the rears to lock up too if that was the case.|
There may be other reasons for this symptom, but certainly one possible cause is the flexible hoses to the front discs. With age these break down internally, collapse, and then work like a one way valve and restrict the flow back away from the callipers so they seize on. Not necessarily visible from the outside.
|What Guy says.|
I had the same problem recently with my brother's MGB.
The rear flex hose was locking the rear brakes on.
|Thought of that Guy but although I haven't replaced them I had no trouble before the old master cylinder spilled its contents so I tend to think that is not the cause.|
|Well if not that, then I understand that some models have an adjustable push-rod at the M/C. If that is wrongly adjusted and has insufficient slack, then maybe the piston perhaps cannot return enough to free off the pressure? But I don't have enough understanding of that issue. |
But given the relatively low cost of hoses, unless yours are fairly new, I would be inclined to replace them anyway. And I would take the opportunity to replace with braided hoses unless you are particularly concerned about concours standards
|As well as what Guy has noted about adjustable push rods - seem to remember something in an old MASC article about different length push rods with replacement M/C's being supplied.|
Do you have the original to compare ?
|Do you have the correct disc brake master cylinder?|
I believe that the drum-brake M/C has a different valve arrangement.
|And Guy's comment about brake lines can not be discarded. |
As the lines usualy appear fine until you introduce fresh fluid.
Then the degrading starts instantanious!
|The internal degradation of the front flexis happened on mine, the bits went into the caliper cylinders and had to be removed. (quite big bits, so big I thought the seals may have been breaking up). Brakes felt rock hard before overhaul due to this. Wouldn't have believed it if I hadn't seen it. Brakes nice 'softer' feely feel now. Like they should be.|
|Dave Squire - Notts|
|Do you have twin bore cylinder. The MGA owners have had all manner of problems with new cylinders by TRW as sold by Moss and recommend AP Caparo. If you have this type of cylinder you may wish to have a look on the MGA threads. You can have your old cylinder sleeved by Past Parts or the like.|
|I've had two combined cylinders re-sleeved by Past Parts. Both leak |
I had your issue with my Frogeye. It was caused by the incorrect adjustment on the piston. It wasn't allowing the fluid to drain and release the pressure. There is a hole in the base of the cylinder.
Your Mk2 will doubtless have the combined m/c.
|Yes it is the combined type and it is a TRW (but not from Moss although that makes no difference). I will look at the MGA threads thanks Alan. I will recheck pushrod slack but am sure its OK and the rods are the original (you don't get new ones with the m/c). I am positive its something to do with the m/c as the brake lights stay on indicating pressure at the switch so I think that rules out the hoses. |
The old m/c has been sleeved but I got a new one as the last lot of seals have lasted less than 3 years.
To elliminate the hoses, slacken off the brake pipe from the master cylinder. If the back pressure relieves itself via the loosened pipe, then the problem is in the master cylinder. If it persists and the brakes are still stuck on, the problem is down the line.
|Trev I seem to remember that it is using the old pushrods on a new cylinder that causes the problem. |
I believe the newer pushrods are shorter and allow the proper displacement inside the cylinder so the relief hole is opened.
And it is still a good idea to get new flexible hoses if yours are oldish.
|Well, there you go. Brake lights staying on is the missing clue!|
Withholding evidence is a criminal offence, Trev!
had the opposite problem with my mc. could not bleed mine when i removed cylinder and reconditioned it. and it was the adjustment of the pushrod.inside the cylinder is a hole which allows the fluid to push on brakes and then release again. if piston not in right place this will happen. try adjusting rod one way and see if it makes a difference if not the other way.
|I have had to return one resleeved slave cylinder to Past parts when it leaked. The fault was rectified and that slave is in service now. I have had other cylinders re-sleeved by Past Parts with no problems.|
|I run Silicone in the one M/C so the leaks have not caused any damage. In the other M/C I run mineral and that has made a mess. This particular M/C was returned to them when I first discovered the leak. It apparently had a burr in the cylinder. Upon return it it leaked again. A job for me now to take it out again, get it fixed and respray the affected area. PITA.|
|Thanks for all the suggestions guys. It won't get looked at till later next week now as I'm working long shifts the next few days. I will try pushrod adjustment first and see where that leads. |
I have looked at the MGA threads and wish I had seen them before I ordered this one. Annoyingly I almost ordered an AP Caparo one from David Manners but as the postage was steep went elsewhere.
Guy - not deliberately withholding evidence, just early stages of dementia! I didn't notice the brake lights at first - that came to light later.
|Trev, Like checking on indicator lights at a pantomime then - They are working, no they are not, oh yes they are, oh no they're not . . . .|
I suppose for brake lights someone has to shout out "Its behind you!"
|bub boom Guy. lol.|
Just one small heads up make sure you get/have the correct 3/4 bore MC and not the 5/8(mga&drumbrakenspridgets)
|Onno definately have the 3/4 bore. Was already aware of the difference.|
This thread was discussed between 17/01/2013 and 19/01/2013
This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.