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MG MGA - MGA brakes

MGA/ Magnette & T type brakes
A recent job prompts me to share some hard learnt info. The rear brake cyl's have the pistons at the top as do the front brake rear cyl's where-as the front cyl pistons are at the bottom. Air can be trapped above the inlet hole at the bottom as the cup sits some way above the hole. So before fitting the brake shoes after fitting the cylinders put a cable tie around the front pistons to hold them in (it pays to do this before fitting the cyl's to the back plates) then push the cup in the rear cyl all the way to the bottom with a socket or something similar & put a cable tie around them (not too tight) And then bleed the brakes. The same result can be got by letting the fluid flood out of the cylinder before fitting the cups but this is messy. You could also bleed the brakes and then remove the piston & with a hose on the bleed nipple push the cup down. You will be surprised how much air comes out.
Garth
Garth Bagnall

Garth, I don't think it matters too much where the inlets to the cylinders or indeed the bleed nipples are positioned, provided the brakes are bled properly. When bleeding, the nipples should be opened enough to get a good flow of fluid out on the fast down stroke of the pedal, and closed before the slow up stroke (train your pedal operator or wife well in this procedure!). I prefer the old fashioned method of brake bleeding and don't trust pressure bleeders. I never have trouble bleeding all the air out of the brakes, but the clutch is a different matter. I have never managed to bleed the clutch in the conventional way, ie using the pedal, I always back-bleed it by pushing the slave cylinder piston in.
Lindsay Sampford

Hi Lindsay

I too am a traditional bleeder without problems (as were the artisans at Abingdon, I might add!)

Brakes bleed very very easily because when they are adjusted up correctly then the pistons are forced into the cylinder bores and thus all air is expelled.

As you note the clutch slave does not have this adjustment and will ALWAYS require the piston to be pushed into the cylinder to expel the trapped air. I get particularly frustrated at the lengths people will go to to find clever ways to bleed systems instead of simply doing the basics correctly!

IMO
Bob Turbo Midget England

I personally am not a bleeding traditionalist, although I agree its not a great problem but using a pressure bleeder does makes it an easy one man job
Vin Rafter

I find that bleeding the brakes gives me an opportunity to speak to my wife!
Lindsay Sampford

Same here Lindsay, I always knew that getting married would pay off eventually!

Having said that, I would have thought that a happily married couple like you and Linda would have found other occasional opportunities to use the expressions "In!", "Out!" and "Hold it there!"

Have a great Christmas All

Colyn
Colyn Firth

Barry Now I know why getting the air out of the clutch slave cyl. has always been a pain in the ++++
Mine has never felt quite right even though the clutch clears ok. Christmas morning or not I'm getting under to sort it ! Sean
S Sherry

A lot of people seem to have the misconception that the MG clutch slave cylinder will trap air. Well, it doesn't. While the bleed nipple is on the side and points somewhat downward, internally the bleed port is at the top of the cylinder, so the air comes our first. See a very clear picture of that internal port here: http://mgaguru.com/mgtech/hydraulics/ht106.htm

The reason the clutch circuit is a bit tricky to bleed is because the pipe is larger bore than the brake pipes, and the clutch circuit runs entirely down hill from master cylinder to slave cylinder. If you don't push fluid through fast enough, or if you use short spurts and pauses, then air bubbles can float upward in the pipe and may never be expelled.

The pressure bleeder overcomes this issue by pushing fluid through in a continuous flow so the air is carried forward along with the fluid. Reverse also bleeding works as the air is pushed upward into the reservoir.

Conventional bleeding with two people can also work well if you're quick about it. Open the bleed nipple and place your finger over the end to serve as a check valve. Have your helper do three or four successive pumps of the pedal with no pause in between, and the job is done. The only trick is to let the fluid flow through so quickly that the air has no time to bubble upward in the line.

To avoid a big mess, put a hose on the bleed nipple, hold your finger on the hose end, and hold you hand in a larger catch vessel. Or, place the hose end in a jar with a little fluid and the hose end under the surface. With quick successive pumps of the pedal, fluid in the jar serves as the check valve, and this can be a one person job (same for brakes or clutch).
Barney Gaylord

Barry, now I know why you are known as the MGA Guru . I now wonder how many gearboxes have been suspected because the slave had not been expurged properly. This is analagious to my dramas with the remote on my G/box. Once I had the short noise end bushed it shifted properly. After three 25th birthdays I'm still learning ! Sean
S Sherry

This thread was discussed between 24/12/2010 and 28/12/2010

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