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MG Midget and Sprite Technical - MASTER BRAKE CYL 76 1500 - HELP !

I have "issues" with my (76) 1500 master cylinder. Back in May I completely rebuilt the Master Cylinder. It had been sitting for 10 years and thus I bought a rebuilding kit. The inside cylinder was a little rusty in places, but not bad. I did not see any pitting so I just cleaned it up and put in all the new hardware from the kit. Along with the master I also rebuilt the front calipers and rear wheel cylinders, which looked good also - no pitting. The brakes seemed to work well. When I pushed the car my wife put the brakes on and all stopped. Pedal felt good. Now, 7 months later after I put the engine in ..... no brakes. I bled and bled them. Air is not the problem. What is weird is that when I push the brake pedal I see that brake fluid is squirting up from the hole in the bottom of the rear reservoir. If I pump the brakes a few times the pedal gets hard and the front brakes will work, but the rear will not.

I then pulled the MC and took it apart. I noticed that it has a screw connecting the 2 halves of the pistons (pic attached). By varying how much you screw the screw into the piston this will change the length of the piston, and possibly be causing my "squirt problem" and causing no fluid to go to the rear system. Anyone familiar with these? How much should I screw the screw into the piston? Thanks .........AL


AL MILLER

Al,

I can only guess, since I have never seen that arrangement. I'm surprised the instructions aren't in the manual.

But, my guess is that it is screwed all the way together. The screw does not look strong enough to take a load, so I think the ends of the piston should butt up to each other to take the load.

Hopefully someone will come along who actually knows.

Charley
C R Huff

Charlie -
I've discussed all this with Al. I know of two other types of interconnect link on the pistons, the "bent cotter pin" that is staked in to one piston and pinned in the other, and the type with the link pinned at both ends. This screw arrangement is new to me, so I suggested he post it to see if anybody else has met one.
It would require adjustment hence instructions for the procedure, and would also appear to need a locknut, which is missing.

The screw/link does not carry a compression load, working only in tension as a locator for the secondary piston, as do the usual links. And it most certainly does not get screwed together tight. That appears to be more or less what Al did incorrectly on his rebuild. The compression load between the pistons is carried hydraulically except in failure of the primary circuit, when the primary piston physically contacts the secondary.

This is the relevant part of my comments to him:
The point of this arrangement is that it allows the
front piston to operate independently of the rear, but pulls the rear back to the correct alignment with the ports when the brake is released.
The fill ports have to be just clear of the main seal lips and on the "live" side of the circuit when the system is at rest. This lets fluid in to compensate pad wear etc., and also allows pressure equalization
from temperature changes etc.

So, I would real;ly like to know if there are more of these and if anybody has the instructions for setting them. I know how to set it, and have told Al, but I'd like to see the "official" version.

FRM
FR Millmore

Al,

I've never seen an arrangement like that either. I have the 'bent cotter pin' setup.

I measure 2-5/16 between the flanges that the opposing cups rest on. The link is loose. It limits the length of spring extension and is free to compress all the way to solid contact, if needed.

I'm in the process of rebuilding mine now, as I've had a leak past the seal onto the pushrod and then my feet. Tonight I discover that the clutch master is leaking, too.

I love the smell of brake fluid in the morning. It smells like victory.

Lee


Lee Fox

Lee -
In other pics Al sent me, it is clear that the screw is free to retract into the primary piston, just like the links do, so it functions the same way. The parts all look factory, not homemade. The other odd thing is that Al's primary piston is black oxide, not chrome plate. I'm thinking it is maybe an aftermarket rebuild with new pistons made to get around the common broken link problem - no links were ever available AFIK. I've made some but it is a PITA. Most odd.


FRM
FR Millmore

Just a thought, is there anything to stop the screw from unscrewing when fitted?
Dave O'Neill 2

Guys, thanks for all the input. Glad to see that Lee is active again. He has helped me a lot in the past. If I screw the screw all the way in I achieve Lee's distance. In the attached pic I measured from the 2 white marks on the piston, also shown where the arrows are. I get 2 5/16 inches like Lee said. This is assuming that we are measuring from the same flanges. Now, I discovered that I have the 2 spring and trap valve items as described by #19 in the Haynes drawing. Could these be a problem? They "look OK", put I don't know if they are working OK or not? Should I remove them? It appears to me that the rear piston needs to be right in the middle of the hole where my toothpick is in the pic at rest. This would allow for fluid to flow from the reservoir into the rear chamber.
When I put the cylinder together the first time I can't remember if I screwed it in all the way or not. (Might of been consuming ale at the time-very hot in Miami in May). As always, your comments welcome.....AL with the non-stop midget.


AL MILLER

Al,

Thanks for the kind words.

The 2-5/16 measurement is between the middle cup and the far right in your picture, not as you have indicated. See my edit of your pic below.

Also, there is a spring and metal cup between those two rubber cups, where the screw link is.

If all else fails, the Victoria British catalog arrived yesterday and the master cylinder is on sale.

Lee


Lee Fox

And as I said, the two main seals - open to the left in the pic - have to be just behind the fill ports, which are the two round holes in the reservoir mount pads. Those are the step drilled holes that are very small on the bore side, NOT the angled or toothpicked holes.

FRM
FR Millmore

Lee,

This obviously makes it much longer than I had it, but so be it. In answer to Dave's question, no, nothing to lock the screw into. However, I would not think that the cups would turn inside the cylinder?
AL MILLER

Well, the spring and cotter pin arrangement is familiar to me, I think from late 60s Saab 96 V4s. As I said, this is a new one to me, and so I'm over my head here. The only thing I can add is that the existing master better have a real nice bore to be worth the trouble to figure it out.

Charley
C R Huff

Al-
Please pay attention - I spent a couple of hours telling you how the thing works, what it needs, and how to do it - read. That includes both the adjustment distance and the whole matter of the screw.

" I would not think that the cups would turn inside the cylinder?"
My original email said "It looks to me like maybe the screw has been broken off at the end. A screw in such an app should have a locknut to keep it from moving - both for the adjustment and because it will wear out the threads, causing failure. "

The cups and pistons don't have to turn to lose the adjustment, only the screw - and it will. Whenever the spring load is taken off the screw it is loose - that would be every time the brakes are applied.
If you can't figure it out, the locknut goes against the face of the secondary piston where the screw is threaded in.

FRM
FR Millmore

This thread was discussed between 08/01/2010 and 10/01/2010

MG Midget and Sprite Technical index

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