Front Master Cylinder Rebuild

[rubberfingers]

Hi

I am in the process of renovating my front MC as it was weeping slightly. I have a rebuild kit but had an issue when I removed the old parts - they sprung out so fast I did not catch them in time to see the order in which they were fitted!

The parts diagram is hard to read for the ordering on the parts and its not clear if the spring fits with the wide or narrow end into the cylinder first. Also I am not sure which of the 2 new rubber washers needs to be stretched to fit onto the middle of the metal plunger and which goes on the end.

Does anyone have an exploded photo of one I can use as a reference please?

cheers

Andy

[kpke]

I just did an FZR front master rebuild. I think most of them are about the same but not 100% sure.

Maybe this will help.

The photo on the top are the parts as it was disassembled.

[rubberfingers]

Hi

Thanks that helps - where does the thin washer go though?

thanks

Andy

[rubberfingers]

Fitted my rebuilt and repainted master cylinder only to find the usual issue of building up pressure is not working. I tried all the tricks, reverse bleeding, vacuum pump, holding the mc at 90deg, leaving overnight with lever tied all to no avail. All I managed to do was get brake fluid on the mc and some on my newly painted wheels taking some paint off

I also rebuilt my calipers using new seals and stainless tell pistons, new braided lines and a bleed nipple banjo on the M/C.

I think there is a weep at the newly rebuilt master cylinder under the rubber boot which I suspect is causing the lack of pressure to build up. Has anyone else had issues rebuilding a mc not sealing correctly again afterwards? I cleaned and polished the bore before fitting the new seals and made sure it was 100% clean before using red rubber grease to fit the new kit.

If I cant get pressure and the weep does not stop I think I may have to buy a brand new MC

Andy

[Speed Freak]

In which direction did you put in the seals?
The "open" side has to point to the fluid side, otherwise they will not seal.

[rubberfingers]

Hi

That diagram is amazing

Alas it was however how I assembled the seals but it still caused air to leak past them. I am sending back for a refund and going to purchase a new MC. I could not see any wear on the bore and I cleaned and polished it well before reassembly but it would not seal. Its one area of the bike I dont want to take any chance with.

thanks

Andy

[pkay]

When you bleed a totally emptied system in my expereince you need to get the vacum up to the banjo bolt on the MC or else it just sucks air back in. I usually cover everything in rags with the banjo and washers and pipe ready to go on one side. Block the whole with your finger to get the pressure up and when you start getting fluid through stick the bolt in and tighten. If you've assembled the MC correctly you'll feel the suction against your finger when yopu use the lever. A mechanic told me this after I had spent hours on another bike - I now do this and works everytime. With 50 odd bikes I get lots of pratcice!! I also use one of those vacum pump sets on the other end to suck the fluid through. Makes it an easy one man job.

[Speed Freak]

Another easy way to get the air out is to pump out the brake pistons in the calipers and push them back in.
You get a big amount of fluid fast thru the brake lines and also in the right direction (up, where the air wants to go).

But if you push the pistons in too fast the brake fluid will spurt out, so please cover your reservoir if you do it this way

I want to have something like this (positive pressure):
https://www.youtube.com/watch?v=btZJKxIib20" onclick="window.open(this.href);return false;

[rubberfingers]

New master cylinder arrived and fitted. Brake lever has never felt firmer

Andy

[ChequeredFlagRacing]

Thanks for posting this image
Just got mine apart as doing a repair kit due to a little weep and the seals are the wrong way around!!!
Strange thing is the brakes were stopping the bike OK and the lever was firm

In which direction did you put in the seals?
The "open" side has to point to the fluid side, otherwise they will not seal.

[JonW]

That is a superb diagram Speed Freak. The CAD project has provided some wonderful output. well done mate.

Ive rebuilt a lot of these over the years. Sometimes a small hone helps, but generally they work fine. Bench bleeding can help but isnt always needed with small units like this.

Since new units are available for sensible money its easy and affordable to grab them. If the new lid is the modern type just fit the old one for originality.