Master cylinder diaphragm, to cut or not to cut?

[idunno]

I have just topped up my clutch master cylinder, i have read some threads saying the diaphragm serves no purpose and should be cut to allow more fluid into the master cylinder, and other threads advising against cutting it. Does anyone know for sure which procedure is the best, or is it just a matter of opinion.

Thanks in advance for any help given.

[topman]

I don't think cutting a diaphagm is a good idea, they are there for a reason. it would be strange to advise cutting it.

[T-Cut]

Originally Posted by idunno

I have read some threads saying the diaphragm serves no purpose and should be cut to allow more fluid into the master cylinder

The diaphragm serves to isolate the fluid from the atmosphere. It also allows the fluid to move out of and back into the reservoir without creating pressure changes in the (sealed) reservoir.

- - and other threads advising against cutting it.

- - - Does anyone know for sure which procedure is the best, or is it just a matter of opinion.

It's not really a matter of opinion.
It's a judgment to be made considering the state of your particular system. Some people cut away the diaphragm because their cylinder is showing classic signs of disintegration, due to seal wear. Many clutches suffer bad operation after a while and fluid loss is also common. The situation can sometimes be improved by topping up the reservoir and bleeding. However, it's not that convenient, so increasing the fluid capacity is an obvious way of reducing maintenance of the fluid. Cutting the diaphragm exposes the fluid to the air, so it must then be regarded as a service item like the brake fluid (change every two years). This would be no bad thing. In fact it's arguably better than a sealed 'for life' system that doesn't live up to the claims.

You have to assess whether your master cylinder is causing more work than you want and whether increasing its capacity would, on balance, be the better option. In my personal opinion, the sealed for life system is inherently flawed and a master cylinder with an easily accessible reservoir (like the brakes) would be better. The clutch hydraulics would then be serviced at the same time as the brakes.

TC