MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - Is it the master cylinder?

For a few months I have been suffering from dodgy brake syndrome. I first put it down to a duff rear cylinder as the seals were split. I duly replaced this a couple of months ago and after several bleeding sessions I was getting a nice firm pedal. However the system still seemed to be gaining air despite further bleeding sessions. My trip to and particularly from the Ace on Tuesday night was particularly interesting going from a couple of shoves to four needed to get decent brake pressure by the time I got home!
I have checked all the lines and I'm fairly certain the flexi pipes are all OK. I'm becoming suspicious that the fault may lie with the master cylinder. I have the oe tin Lockheed cylinder which I would prefer to keep as I believe these are now nla. I seem from time to time to be gathering brake fluid in the rim of the cylinder suggesting the cap may be leaking but do the caps not have a breathe hole in them in which case would this make any difference and cause air in the system? I've noticed that the paint on the inner wing around the pedal box is wrinkled, again pointing to fluid leaking somewhere from the mc. I've just taken the cover off the pedal box and the area around the piston does not seem to be leaking and the fluid in the m/c is still quite high so if it's leaking, not much is getting out but the brake lines appear to be full of air.
I'm going to take the brake drums off at the weekend just to check the leak isn't at the rear wheel cylinders again but failing that anyone got any ideas. If the m/c doesn't appear to be leaking at the piston could the seals still have gone and be causing the ingress of air?
If so, is a rebuild of the m/c with new seals relatively straightforward assuming the bores are in good nick?
Advice as ever welcomed before I end up taking the whole blasted system apart! Is there any definitive way to check whether the m/c is working as it should?
Matt1275

If you have rebuilt on of these MC before or seen it done, then yes its straight forward, but for.everyone else... its a 3 day adventure on gilligans island....and haynes manual is just going to laugh in your face

Prop
Prop and the Blackhole Midget

It does sound like your master cylinder could use a rebuild.

As long as the bore isn't badly scored or corroded, you can hopefully get away with a seal kit and a deglaze.

Single-circuit cylinders (all UK-spec 1275s) are easily dismantled and rebuilt, unlike dual circuit M/Cs.
Dave O'Neill 2

The only tool you need to make life easier for this job is a good pair of internal circlip pliers. If you have a camera, it's handy to take a picture of the internals in the order they come out to ensure that you can put them back in the same way. To hone the bore I used a piece of scrap rod, split lengthways halfway down with a hacksaw; into which some wet and dry paper was inserted in a spiral around the rod. In an electric drill, with plenty of lubricant in the bore, nearly all the imperfections were quickly removed to produce an almost new finish.

In the picture, the parts have just been removed from the MC and the old seal is still on the piston. I soaked the new seal in hot water for 10 minutes to make is soft enough to be easily put on without damage.

I also cleaned the work-top and all the parts prior to reassemble to make sure no grit was picked up.

The MC was stripped of old paint, undercoated, painted and lacquered.


Nick Nakorn

Here's the MC with paint.


Nick Nakorn

Many thanks Nick, That doesn't seem too bad a job. Are all the rebuild kits from the usual sources all much of a muchness in terms of quality?
Matt1275

Matt
When you bleed the brakes you say the lines are full of air Does this mean you are getting air out of all the wheels or just some wheels ?
This would give you a guide as to where the air is getting in If it's only at one wheel it would point there
I had a disc caliper on an MGB once that didn't show any signs of leaking but kept getting air on that one wheel. Took it apart and fitted a new piston seal and it was fixed
willy
William Revit

Are you sure the lines are full of air, or is it just that you have a soft pedal?
Dave O'Neill 2

Nick

I LIKE THAT...very cool idea for honing the cly.

The major issue I alwas had was that idiot spiral clip and then forcing the spring into place while trying to reassemble

That was on the duel circuit for the 71 usa model

Prop
Prop and the Blackhole Midget

Never tried honing one of these. How far can you go without taking too much out of the bore? How good are the seals at sealing in a slightly oversized bore?
Lawrence Slater

Lawerance

Ive never honed them ither.

My understanding its like doing a engine cly. Your just breaking down the glazing and brightening the metal a bit...nothing radical or extreme

Prop
Prop and the Blackhole Midget

Dave,
I'mm 99% sure they're full of air as I can pump the brakes up. I'll try a bleed at the weekend before I do anything else to check. Could it be as simple as just a dodgy Master cylinder cap as this does appear to be leaking, whereas I can't see any evidence of fluid coming out via the piston into the pedal box. As I mentioned, fluid loss in the m/c doesn't appear to be significant as its still pretty close to the top, more its a case of air apparently getting in somewhere. I did drain the whole system down a couple of months ago to renew all the fluid (still Dot4 though). Should I have bench bled the m/c then perhaps? I just filled the m/c and bled the whole system through a couple of times with an Eezibleed and all seemed fine until last week.

Matt
Matt1275

Matt, I can't vouch for the quality either way. I've had a break of over 20 years fixing cars full time so it's all new to me again. I'm assuming both Moss and MGOC will be OK but I've not been happy with several items from MGOC so I tend to stick to Moss. Back in the day I'd use whichever brands Unipart or Stockwell Motors happened to have.

Lawrence, if the bore is heavily pitted it's not worth saving given that replacements are still not too expensive. But the seals are a pretty tight fit so small blemishes can be honed away without sealing problems. I've never measured the amount so it's really a matter of judgment by eye and finger. Not a helpful answer but all I can say is that if the damage feels slight to the touch then hone it. It's a bit like feeling a bearing journal and knowing when it needs a grind or not.
Nick Nakorn

The reason I asked, is that when I used to bother kitting wheel cylinders, even though the bores looked fine, the kits(original lockheed or girling) didn't last long. Better off with a new wheel cylinder irrespective of cost. But I didn't attempt to hone the bores, just cleaned them.

I last changed my Sprite master cylinder many yonks ago after kitting it because of fluid leaking past the seal. The bore looked smooth with no pits, but the seal kit didn't last long. Again all I did was clean the bore.

Now I have a small weep again. True the master cylinders aren't that expensive, but if a hone can make a seal kit work, I might have a go too.
Lawrence Slater

I have the drill set to a medium speed and do a few seconds (say 5 to 10 seconds depending on how pitted the surface is)with a 240 grit and then about 10 to 20 seconds with a 1200 grit with lots of oil!. I think many rubber seals fail because the cleaning /honing is not rotary and so there are longitudinal faults that cannot seal. After the honing all the crap needs to be washed out with carb-cleaner or Jizer/Gunk and/or compressed air, and/or a power washer. I have a parts washer so it goes in there and then is hosed out and dried with air. I've switched to silicone fluid so I use some of that to reassemble. When all sealed up again, I had the units in plastic bags prior to refitting. BTW, in the past I've used silicone spray for fitting up but there's something in the propellant that makes the rubber sticky and I now don't use it.
Nick Nakorn

Lawerance,

T
I HAD that exact same problem and it plagued my for the 1st few years... at that time you couldnt get new master cly so I purchesed 3 rebuilts which failed and I rebuilt each one with no success

The rubber parts kept turning into black gooy bubble gum...almost lime the seals where melting, or wearing out quickly...castrol LMA dot4 helpped the most

Finally I went to tilton and got a 3 peddle assembly and cant be happier


Prop
Prop and the Blackhole Midget

"Finally I went to tilton and got a 3 peddle assembly"

Accelerator, brake and clutch?
Dave O'Neill 2

Dave....

Hahaha.... and yes, roflmao

Great one! I did not see that one comming.

Prop
Prop and the Blackhole Midget

This thread was discussed between 12/09/2014 and 13/09/2014

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.