do master clylinders have to be airtight - Triumph Forum: Triumph Rat Motorcycle Forums
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post #1 of 8 (permalink) Old 11-05-2012, 05:00 PM Thread Starter
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do master clylinders have to be airtight

This is a bit theoretical. Unable to unscrew the brake master cylinder screws, l thought about drilling a hole in the top or side of the reservour and syringing brake fluid in. Then pluging the hole with a small screw. As the hole is above the fluid it need not be air tight. My friend doubts this but whenyou bleed the brakes the top is off and the brakes still pump fluid. As air compresses if there is any pressure above the cylinder pump , the fluid would rise in the reservour but it doesn't. l don't understand what the membrane is even there for, if it had a hole in it would the brake still work. Please enlighten me in case l kill myself on the first emegency brake manouver.
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post #2 of 8 (permalink) Old 11-05-2012, 05:34 PM
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Just drill the screw heads off, lift the lid & unscrew what's left.

Air must be able to enter above the membrane - it has to, to allow different fluid levels. The membrane helps prevent contamination from the air, esp moisture being absorbed into the hygroscopic fluid. You will find small slots in the lid mating surface which allows air flow above the membrane.


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post #3 of 8 (permalink) Old 11-05-2012, 09:14 PM
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If you look reeeeeally carefully at the side of the master cylinder, you'll see a tiny pinhole, which weeps a little brake fluid. Has to be there so you don't get a vapour lock (is that the right term?) in the line, I think.

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post #4 of 8 (permalink) Old 11-06-2012, 01:59 AM
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Master Cylinders

As Mike describes must be air tight below the rubber membrane. When the brakes (or clutch) are bled correctly there is no air in the lines between the piston in the master and caliper pistons so a vapour lock cannot occur provided that the correct fluid level is maintained.
There is definetly no small weep holes in either of my masters, if there was, I would be worried.
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post #5 of 8 (permalink) Old 11-06-2012, 02:14 AM
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Undoing M/C screws

Quote:
Originally Posted by thekeyman View Post
This is a bit theoretical. Unable to unscrew the brake master cylinder screws, l thought about drilling a hole in the top or side of the reservour and syringing brake fluid in. Then pluging the hole with a small screw. As the hole is above the fluid it need not be air tight. My friend doubts this but whenyou bleed the brakes the top is off and the brakes still pump fluid. As air compresses if there is any pressure above the cylinder pump , the fluid would rise in the reservour but it doesn't. l don't understand what the membrane is even there for, if it had a hole in it would the brake still work. Please enlighten me in case l kill myself on the first emegency brake manouver.
these screws can, over time, become very difficult to loosen (steel into alloy). The trick is to use a steel punch that fits over the screw head (does not contact the alloy body) and tap the punch with a light engineers hammer several times. Usually does the trick.
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post #6 of 8 (permalink) Old 11-06-2012, 05:03 AM
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The screws are made of cheese.Drill the heads off and the rest will come out easily with a mole grip.
Replace with stainless steel and use a little coppaslip on the thread.

Tapping the screw before you try removing it does help.

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post #7 of 8 (permalink) Old 11-06-2012, 11:29 AM
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All good advise. One more thing: most folks try to use a Philips driver. these are not Philips screws ad the driver will not fit correctly, hence the damage to the screw head. the correct tool is a #2 JIS driver. This fits tightly even in most damaged heads and generally solves the problem.

Philips drivers and screws are tapered and are designed to cam out under excess torque. The JIS driver has straight sides and grips firmly.

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post #8 of 8 (permalink) Old 11-06-2012, 01:42 PM Thread Starter
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used a jis2 bit and even bought a damond coated bit to no avail. Drilled the heads ok but required expert use of a thin chisel to pry off caps. When off the screws were loose ,wierd!!
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