Look out for pits in the master brake cylinder

[2WheelFun]

Rebuilt my 7200 mile master cylinder on my 79 a while back only to have weird brake feel. It was good lever then if you relaxed the grip slightly and applied pressure again it would bleed down. A couple of bleed downs and the lever was on the grip. I took it all apart to see if something was put together wrong and all the seals were installed properly. Deep in the bore had some pitting that I didn't notice before hand.

My 80 parts bike had its master still so I cleaned that one up and the bore was good. The cup is all weathered and crusty. They changed the master slightly from 79 Special to 80 Std so my nice cup wouldn't work but I do have good brakes now. I do not like the plastic cups, they seem prone to leakage.

[dbbrian]

I bought the stainless steel cylinder from Sirius Consilidated after looking around. Price was the best and they were the only ones identifying the correct diameter Dimensions
Outer diameter 38.10mm

https://www.siriusconinc.com/pro-det...duct_id=120653

[2WheelFun]

I'm talking about the bore inside the aluminum master cylinder, it was pitted towards the bottom of the hole. Inspect all your parts with a keen eye when rebuilding them!

[jetmechmarty]

I do not like the plastic cups, they seem prone to leakage.

You did remove the plastic reservoir from the master cylinder? Did you replace the packing (o'ring) on the bottom? If you didn't, that packing is over 40 years old. It's no good. I wish I had the specs on a replacement, but I'm afraid I don't.

[2WheelFun]

Yup, the O-ring was replaced. I used one out of the harbor freight assortment if it didn't come with the rebuild kit.

The thing is that you lube up the ring to press it on and it weeps on to the freshly painted master. The old O-ring was probably why the master was dry, pitted and full of crystal when I bought the bike. Saw the same failure on my brothers 750 which is now mine. The plastic cups look like crap when they age.

[jetmechmarty]

The plastic cups look like crap when they age.

Somewhere in all this data is a thread on restoring the reservoir. I believe it was posted by John.

I found it! https://xs11.club/forum/idle-talk-fo...-detailing-tip

[2WheelFun]

Thanks for the tip! I will have to try that when I give the ol girl a good detailing.

[Radioguylogs]

You did remove the plastic reservoir from the master cylinder? Did you replace the packing (o'ring) on the bottom? If you didn't, that packing is over 40 years old. It's no good. I wish I had the specs on a replacement, but I'm afraid I don't.

The Special O-ring is a different than the Standard O-ring.

When I did the Venturer, I bought good replacement O-rings for a Standard from McMaster-Carr. They come in lots of 25 pieces, so if anyone needs an O-ring for a Standard, I can help.

[2WheelFun]

I've got the standard master on there now and I used the O-ring off the Special. It was the special master that had the pits in it. It's holding but there aren't many miles on it as of now. If you know the dims, I could double check to see if the one that's in there is correct, I know it's metric.

[Radioguylogs]

Sorry, I measured it and ordered a replacement based on my measurements.

I don't have any record of the size I ordered.