Accessing master brake cylinder

How is this accessed? It has been suggested to me that you need to remove some fabric from the belly as it is not possible to remove it from above. This seems a bit extreme for an aircraft which in theory can be worked on in the field?

You will have to cut fabric only if you are going to remove the brake frame assembly.... The master cylinder itself is removed by the 10 screws on the front of the frame/pedal assembly on the inside of the aircraft...

Brian

I just had mine out this spring. As Brian said just remove the 10 screws on the front. It helps to have a very long skinny shaft philips to remove the bottom ones. It lets you flex the shaft for better access to the bottom screws.
DENNY

when you have the screws out replace them with allen head screws. Use a ball driver to reinstall them and it will be much easier

And if you leave out the bottom one, next time it will be a ten minute task. Twenty, if you make your own diaphragm.

The first removal will require a really, really good long Phillips screwdriver. I don't even try without a Snap-On screwdriver.

bob turner said:

And if you leave out the bottom one, next time it will be a ten minute task. Twenty, if you make your own diaphragm.

The first removal will require a really, really good long Phillips screwdriver. I don't even try without a Snap-On screwdriver.

Click to expand...

I've just bought some 3mm Viton rubber to make up a diaphragm. I measured the thickness of the outgoing (and well ripped!!) old one and it was circa 3mm in the uncompressed areas, is this what others have used? What is the best tool to cut the circular shape? I'm planning on using a sharp craft knife at the moment. And once in and working, what kind of life should a diaphragm give?

Does leaving out the lowest screw affect the leak resistance at all? It would certainly have made things easier not having it, as the only reason I had to take the seat out was to get access to loosen off the pipe fitting nut, as the lowest bolt was RIGHT behind it!!!!! :roll:

Thanks!!

Philly

Oh and by the way, while working on a problem on the other master cylinder earlier last month, I was able to remove the port master cylinder frame without removing any fabric by using a small socket wrench and some careful dexterity (my hands aren't massive) IF I ever take fabric off underneath in the future, some captive nuts on a plate for both master cylinder frames will be high on my "useful mods" list

Why not just buy the correct parts from Univair?

Scott Master Cylinder

Having stripped the unit down, probably best to re-assemble it with the correct certified parts including new screws. Univair will post a pair of diaphragms to you here in the UK within days.

For info, the following service bulletin suggests a 200hrs life and the Univair labelling confirms that.

Why mess with the old diaphragm to begin with and just update the things with newer high pressure boosters..??

Any brand booster is better than the diaphragm IMO..

Brian


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Well, doing that for just shy of a half century, I found that I could get easy 200 hours out of a truck tube. There has to be better rubber, but I just had that giant slab laying around. We have more recently used Buna N, but since my masters are all Grove, I am only in the business for friends.

I started leaving off the lower screw in the 1970s. Understand, I fly pretty much every day, and had no problems. The allen head screws came after a particularly difficult Super Cub diaphragm change. That was where I learned about $25 screwdrivers.

I agree that ordering from U is a good idea - they are producing stuff we really need, and we should support them. But I found no difference in longevity. I did start using Dot 5 in the 1980s, and that may have made a difference (it is inert - it attacks nothing!). Dot 5 is now way too expensive for Cub brakes; I may convert to 5606 next time I need a $50 quart of fluid.

And I agree with Brian - except I would add that a reservoir master and a tight system means brake woes are over. We check the fluid when we replace the pads, and the pedals are always - always - in the same place!

Oh - cutting rubber: I use Wiss-7. And I have a leather punch for the holes.

bob turner said:

Oh - cutting rubber: I use Wiss-7. And I have a leather punch for the holes.

Click to expand...

Thanks for this Bob. In the end we we used a scalpel to cut the shape, using the master cylinder part as a pattern and an air tube drill to make the holes, again using the MC part as a pattern (leaving out the bottom hole as suggested in this thread ), although I could have borrowed a leather punch from my horsey friends also.

I completely agree with buying from Unvair to keep them in and interested in the business of helping us etc and I've already spent quite a chunk with them, which is then further inflated by UK shipping and import duties and VAT etc. But as these components really are just shaped Viton rubber sheet, it made no financial or operational sense at all to buy them pre-made, plus the shipping/tax costs and shipping times etc.

The shipping costs really are horrendous and to give an idea, shipping a <$200 PA12 windscreen almost triples the price once landed, making it an EXTREMELY expensive part here, compared to its relatively affordable cost in the US