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MG Midget and Sprite Technical - Master cylinder weirdness

My brakes were fading.
'63 midget, 3/4" bore M/C
Rebuilt MC, it was unbelievably crappy.
See MC gallery here:
http://gallery.me.com/jvandyke#100153
Cleaned and rebuild kit.
Better, but fluid was coming out the vent hole in the cap. A pronounced surge of fluid was there on pressing the brake. (but brake action was good). It seemed to eventually pressurize the reservoir and leak. I tried a non vented cap but it would come out sooner or later anyway.
I have it apart again, cleaned again and will renew seals again. Anyone experience this before? Could just be funny seals as I did swap the main seal from the clutch side to the brake side and it seemed to reduce the spout down to a gurgle. Currently on the bench with the outlets plugged and I can't move the pushrod more than a 1/4" (by hand) before it's rock hard.
Anyone know what the second set of holes in the bottom of the reservoir are for? The smaller ones?


J Van Dyke

JJ.. The smaller holes must be open. They allow the fluid to come back in when you release the pedal. Also they must be clear of the rubber cup when in the relaxed mode. Some cups swell up and block this by-pass hole and then the brakes remain on and drag. If so then a .02-.02 cut must be removed from the piston head.
Sandy
conrad sanders

It looks past the overhaul stage, I would replace with a new/re-con unit.
Alex Sturgeon

New units are available although they are ment for the early drumbraked Midgets and MG A the do a fine job on a later diskbraked car. Pedal travel a little shorter and a bit stiffer pedal but hey the do all fade so a little more travel is ok:-)

Be sure you have the correct lenght pushrods.

Over here new units are 150 euro's.

Bas
Bas Timmermans

I didn't think new 3/4" MC could be bought. The bores feel good to me anyway. I now think it's seal related though.
The ones I got today are a lot "shallower" than the ones that were in it or the ones that came in the kit. I was running one of these "skinnier" seals on the brake side because I ruined one of the ones in the kit. These "off-the-shelf" ones don't have as much of a "skirt".

I'll take a picture.

So, where does this smaller by-pass hole lead too? It sure seemed to me that that is where the fluid was squirting up from when pushing the piston. I'll order up another rebuild kit with proper seals and try again.


J Van Dyke

The second small hole leads directly to the cylinder chamber as does the large one. The squirting is normal. Look at the brake cup-it is behind the hole and the first act is to move forward compressing fluid.Until the cup does pass both holes there is always a bit of fluid popping up. Normally you don't see this as the cap is on.
Sandy
conrad sanders

Neither of my second, smaller holes go straight into the bore. There's metal right under them. I assumed it was a little passage to somewhere else, to the bore yet, but not straight in. I tried blowing air through it to determine where it goes and it seems to make it to the bore somehow, just not straight through like the larger, more forward holes. I'd be interested to know more about these "vent" holes, "bypass" holes, "refill" holes.
It makes sense though that the smaller seal doesn't cover whatever hole this is soon enough, allowing more pressure and fluid to escape the bore than was intended, pressurizing the reservoir and causing leaks. I just need to get the proper seals and verify it. Must order kit again.
J Van Dyke

I would consider have the unit ultrasonically cleaned to remove all the crap from the deep dark places
Alex Sturgeon

It sounds like the check valves aren't holding against the brake shoe return springs and eventually the springs force the wheel cylinders far enough in to push fluid out the cap. You pump up the brakes and force a lot of fluid out to the wheels, then that fluid is slowly forced back by the brake springs. The check valves should keep about 6 psi of pressure on the system to counter the springs and reduce pedal travel IIRC.
Bill Young

Thanks for the comments and suggestions. It's not that the fluid level in the tank goes up enough to reach the cap and come out. It'll come out the cap vent even if the fluid level is very low. I'll shut up and get new seals (proper ones) and see what happens.
J Van Dyke

Those second small holes should go directly into the cylinder. Use a .060 drill and clean out the crud. I have also used torch-tip cleaners to clen those small holes.
Sandy
conrad sanders

Really? Straight into the bore? I swear it was metal and not crude but I'll haul out the digital microscope and prope it!
J Van Dyke

Thats what I thought before but the hole starts as quite large but halfway becomes very narrow. The only thing I could push trough was 1 twine of a bike brake cable. The 1 mm drill I had lying around was to big!

Bas
Bas Timmermans

Interesting. But it does goes straight in then? Just narrows down a lot? How close is this?


J Van Dyke

Nope its not a tapperd hole but a app. 3 mm hole that seems to end blunt but in the center there is a tiny hole that continues straight down to the main bore.

Its so tiny its prone to block and with binding brakes as a cause and sometimes the terrible experience of a pedal that wont return:-(

Bas Timmermans

J. The drawing is mostly correct except that both seals will be behing both hole when the pedal is full back. The larger hole of the two is to allow fluid from the resevoire to enter and keep the cylinder full.

Sandy
conrad sanders

Alright, the drawing is supposed to represent pedal up. But I did it from memory, at work, during a loll in the storm .
I did find the tiny hole hiding in there, got a single strand of multi strand wire to fish through and see it come into the bore. Hard to believe it was functioning when I got the car with all that crap in there. (but it's had about 1500 miles since I went through it the first time). Wonder if I was the first to crack it open?
J Van Dyke

Conrad,

you will find J Van Dyke is right with his drawing.

Mike
M J Pearson

I think what was happening is the main seal I put in was smaller (thinner) and not covering the little hole quickly enough, allowing too much fluid and pressure up into the reservoir too quickly. I need the "deeper" skirted seal that comes in the kit. I think.


J Van Dyke

So, why doesn't the sleeved version have the little secondary holes?
http://cgi.ebay.com/ebaymotors/Sprite-Midget-Spridget-Disc-Brake-3-4-Master-Cylinder_W0QQitemZ180306461616QQcmdZViewItemQQptZMotors_Car_Truck_Parts_Accessories?hash=item180306461616&_trksid=p4506.c0.m245&_trkparms=72%3A54365%3A1239%3A1240%3A1318
J Van Dyke

This thread was discussed between 12/11/2008 and 18/11/2008

MG Midget and Sprite Technical index

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