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Master Brake Cylinder Rebuild ?

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doughairfield

doughairfield

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I've to the master brake cylinder with the .70 bore, denoted by the 2 rings on the cylinder. I purchased what I thought to be the kit to rebuild it. My question is regarding the plunger seal. The old one, which didn't look bad at all, seemed to fit the plunger perfectly. The smaller end of the conical part of the seal was the exact diameter of the plunger. I pulled it off anyway and put the new one on, but it seems to be slightly smaller than the plunger. Is that normal/something to be concerned about?
 
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tdskip

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Do you have a picture of the part in question Doug?

Sorry if I'm being dim, but which seal again?

TR4abrakemastercylinderdisassemb-1.jpg
 
TR3driver

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Well, there are only two seals (not counting the dust boot). The big seal seals against the diameter, so it's diameter is important (and must be larger than the piston since the piston is slightly smaller than the bore).

But the small seal seals against the end of the cylinder, so it's diameter isn't important at all.
 
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karls59tr

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I read with interest your issue with the seal as I am about to do the master cylinder on my TR4A.What company did you purchase the kit from? The flared part of the cone seal would fit the bore and probably seal OK but wouldnt it be a better seal if the smaller part was the same size as the bore? Is this a case of aftermarket parts not being up to the standards(specs?) of the original Girling seals?
 
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doughairfield

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It's the seal that goes around the plunger, the one on the right side of that assembly in your hand. So looking at your pic, your plunger gasket doesn't seem to be quit as big as the plunger just like my new gasket. Maybe my old gasket was the one for the .75 bore and was just installed incorrectly.
 

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doughairfield

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Hi, I purchased my kit from Moss. I triple checked that I had the correct part and the correct master cylinder. I would like to think that the plunger gasket would need to be the same diameter as the plunger but from the pic earlier in this thread, maybe it doesn't have to be, but this is my first master cylinder rebuild.
 
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doughairfield

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Sorry, one other question, what do you recommend cleaning the master cylinder with? Brake cleaner?
 
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bailee2

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I would clean it with a mild solvent and don't let any of the cleaners contact the new seal in case there is a issue of seal and solvent compatability.

Also if the piston is scored chances are the bore is scored. Check with a magnifying glass. If so, you should lightly hone it, re-clean and lubricate the seal before assembly.

The best fix is to have it sleeved by Apple Hydraulics so that it is a reliable again.
 
swift6

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That photo looks suspiciously like a Clutch Master Cylinder. Thread is titled Brake Master Cylinder. Which are we talking about here?
 
startech47

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Do not use brake cleaner, it is intended for use outside of the hydraulic system. Always use soapy water to clean metal surfaces after honing, followed by drying, and a coat of brake fluid to prevent rusting of the honed surface. When honing engine cylinders substitute oil for the brake fluid. Use of hydrocarbon solvents drives the honed material into the metal pores.
 
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doughairfield

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Ok, dumb question but how do you get the rubber dust cover over that washer, or the fork on the push rod. It won't stretch that much for me.
 

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Geo Hahn

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doughairfield said:
Ok, dumb question but how do you get the rubber dust cover over that washer, or the fork on the push rod. It won't stretch that much for me.
Click to expand...

I've obviously done it in the past but have no recollection just how -- when I need more stretch I generally get the rubber bit warm (soak in hot water) and lube the opening a bit (Vaseline might work). Looks like the end opposite the fork is the way to go on this.
 
Andrew Mace

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Right or wrong, I've always put those seals over the "forked" end. I've had one or two aftermarket seal kits in my time where some or all the seals were so hard that they had no "give" at all and split...after which I swore off anything but genuine Lucas/Girling branded kits. Maybe some aftermarket kits are better now than they used to be? Hopefully that's the case, since I understand some kits just aren't available any more from Lucas (like the GT6+/Mk3 dual master cylinder kit).
 
TR3driver

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swift6 said:
That photo looks suspiciously like a Clutch Master Cylinder. Thread is titled Brake Master Cylinder. Which are we talking about here?
Click to expand...
Shawn, the dual circuit brakes didn't start until the TR5/250. Until then, clutch and brake MC were identical (or nearly so, they might have been different sizes in some cases).
 
TR3driver

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doughairfield said:
Hi, I purchased my kit from Moss. I triple checked that I had the correct part and the correct master cylinder. I would like to think that the plunger gasket would need to be the same diameter as the plunger but from the pic earlier in this thread, maybe it doesn't have to be, but this is my first master cylinder rebuild.
Click to expand...
It's normal for the 'body' of the seal to be slightly smaller; only the lip (the part that actually seals) needs to be slightly larger than the bore.

The seals also swell somewhat in use, so it's not unusual for the old seal to look slightly bigger than the new one. For DOT 3/4, the brake fluid naturally makes them swell; DOT 5 deliberately has an additive to make them swell. Although originally unintentional, the swelling helps reduce leaks and prolong the life of the seal.
 
TR3driver

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doughairfield said:
Sorry, one other question, what do you recommend cleaning the master cylinder with? Brake cleaner?
Click to expand...
Brake cleaner is what I use.

Sorry, can't help with the fork question. On a TR3, the fork comes off.
 
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tdskip

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Doug - it will stretch to fit if you go over the pushrod. It was a bit hairy the first time you do it since you worry about it tearing, but the rubber is tougher than it looks/feels.
 
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doughairfield

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Thanks, I soaked it in some hot water for a little while and got it to stretch enough to get it on. Thanks for the help.
 
swift6

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TR3driver said:
swift6 said:
That photo looks suspiciously like a Clutch Master Cylinder. Thread is titled Brake Master Cylinder. Which are we talking about here?
Click to expand...
Shawn, the dual circuit brakes didn't start until the TR5/250. Until then, clutch and brake MC were identical (or nearly so, they might have been different sizes in some cases).
Click to expand...

doh! Would have helped if I had looked at the OP's car type. The photo that tdskip posted looked identical to a couple of TR6 clutch masters that I recently rebuilt.
 
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