• Enter the March CB750 Supply gift certificate giveaway! It's easy... Click here, post something, and you're entered into the drawing!

71 K1 master cylinder rebuild

Vicman

CB750 Addict
Messages
160
Reaction score
0
Points
16
Location
Valdese, NC
So I got the MC taken apart, cleaned the chunked up old fluid (it's been sitting since at least 78) and put it back together. Problem is it won't pressurize or pump fluid. The spring is pushing the piston out very slowly back to the brake lever.
I'm pretty sure I put it back together the way it came apart but this is puzzling me. I do have manuals and have the rubber seals pointing in the correct direction including the one on the piston pushed up past the shoulder. I also have cleaned all the little port holes in the cup at the end of the spring and behind the reed valve at the end of the piston. The return hole is clean and open as well.
I can take a pic of the assembly as I have it but the rubber seal at the spring end is a pia to get back in.

Where am I off? Thanks
Vic
 
Well if the 1st seal was a pain to get in, you probably have it in right... would go in easy if it was backwards. It sounds like you already checked the position of everything and are confident that it's assembled correctly.

At this point I'd recheck the tiny fluid return hole to be sure it's open... I think you already checked that, but it's worth another check and no disassembly required.

Slow return of the lever is kind of baffling if everything is clean and assembled correctly. Any chance the lever doesn't move freely and is actually causing some drag during return? I've never had that problem. but you say everything is clean, so what else might it be? I assume the spring seemed relatively solid and strong.

As for not building up any pressure... have you tried removing the banjo bolt and just putting your finger over the hole, then pumping the lever to check for pressure and maybe help bleed the master cylinder?

You may have checled Youtune already, but in case not, here's a decent review of rebuilding a master cylinder: http://www.youtube.com/watch?v=2W24asyNujs
 
Thank you for the reply!
That was a pretty good vid complete with glass of wine and a cat.
I assembled it just like he did.
Still nothing is making it past the piston to the line for some reason. I guess it's dis-assembly time again.

To answer your points above. The lever is clean and moves freely, it's more like the seals have swollen and won't allow the spring to return the piston. The bore is completely clean, I ran a fine dingleberry hone down it to clean it. The weep holes are pristine clean.
 
Well I wish you luck... and if it's any consolation, it'll be easier the second time. The biggest asset is long snap ring pliers, but it helps to know what you're getting into.
 
Got tired of screwing around trying to get the front seal in so I turned up a bushing as an insertion guide. That worked a lot better and made simple work of putting the thing back together.
The front seal is the one holding up the progress. I suspect when I used the dingleberry hone I made the finish a little too rough for the seal to slide easily on return. I polished it up with 800 grit, now it returns pretty good but still doesn't seem to pressurize as well as I want.
Tomorrow I'll hook it up to the caliper and see if I can get it to pump up.
 
Yeah,
I poured a little fluid down the line hole to see if it just needed a little prime but it was getting late so I'll mess with it tonight and see if it works better.
That bushing trick made all the difference in the world for putting it back together though.
 
Mounted it all up at lunchtime. Not much love. I think the seals are just too soft or something, a little going out the line hole and a little coming out the handle side. I guess it's time for a proper rebuild kit and may as well do a new line assy as well.
 
Nah, I was hoping for the give it all a good cleaning and back on the wheel job but it looks like I may need to replace the 42 year old rubber.
The parts all looked pretty good with no tears or scratches. The spring is quite strong and is returning properly now that I put a good slick finish in the bore. I can't imagine that I took any more than .0001-.00015 out of the ID cleaning it.
I was looking at some re-build kits and they showed the reed spring with a gap behind it, is it supposed to have space behind the petals?
 
Ordered the kit from Dime City today.
Thanks for your insight.

Rode my 73 today. Man what a difference new tires make! I plan to put a few miles on her this weekend.
 
Got the rebuild kit today and will change out all the bits tonight. Dime City rules.
Hopefully I can pump the caliper apart and call this part of the job finished. Thanks for your replies T.
 
WooHooo!:D
I don't know if it was new parts or the 40th time is the charm but all went together like a snap and she pumped up and even opened the poor old caliper with no problems! My little insertion bushing:thumbsup: made putting all the bits back in super simple too!
The caliper pumped out with the chrome being in perfect shape after a good wiping down, I think I'll be in perfect shape before bedtime.:D
 
Back
Top