Brakes... won't brake

[ihavereverb]

Hey guys, so this is actually for my girlfriend's bike (1979 Kawasaki KZ400), the forum for that thing has been dead for like 2 years though, and this place has such smart folks

We bought this bike and I thought it would be easy to get back on the road, and I've been kicking myself in the ass for the last 3 months.

My most recent project/problem is trying to get the front brakes to work. I have a vacuum pump and speed bleeders on there, and I must have pumped through 15 gallons of fluid. I just can't get any pressure, and I keep getting bubbles. I don't see any obvious leaks anywhere, though, which makes me think it might be the master cylinder needing a rebuild?

Is there a way for me to test that theory out without buying a rebuild kit first?

Also, here's a curveball: I had the brakes working a week ago, they were just soft. So I tried to stiffen them up with my vacuum pump (built a bunch of pressure, then opened the bleeder), but that only caused them to go completely soft and I haven't been able to get them back since. If there's a leak or something, wouldn't this have been impossible?

 

[Motogiro]

If you've done a lot of bleeding and you still have air with no braking then I would suspect the piston is seized. If you manually bleed the bike using the master cylinder then you should have some idea to it's operation. Bleeder open and the lever should just sink and have no pressure. Bleeder closed and you should have lever resistance. If that's operational you have a problem in the caliper and will probably want to rebuild it. Since that's a single rotor you want that brake at the top of it's game. Unless you still have a lot of air trapped in the system?

 

[ihavereverb]

Hmm, I don't think that's it, since I was just outside and pushed the piston in pretty easily. Also, I get no pressure when I shut the bleeder, and if the piston was seized I should hypothetically still be getting pressure, just no brake?

 

[Motogiro]

Sounds like the master cylinder is getting blowby on it's piston..

 

[FinalImpact]

Three things come to mind:

If just a few drops of fluid come out after pumping it up and cracking it, the piston in the MC may not be coming OUT all the way. If it doesn't come out all the way it can't get a fresh charge of fluid from the reservoir, thus it doesn't build pressure.

Due to the age of the bike; I'd be rebuilding the MC and WC with fresh seals and putting a new brake line on it. Careful inspection of the MC bore and piston is critical as brake fluid attracts moisture, this moisture can be very corrosive and etch the bore tearing up the piston seals. If this is the case, it may need a new MC.

Another option is the brake line itself is ruptured and ballooning each time it builds pressure.

IMO vacuum bleeding pulls the trapped air molecules apart making the problem worse. Pressure bleeding should work once you establish what is wrong.

Oh - get it to build as much pressure as you can, now feel the brake line all over while pulling the lever. If you feel it bulge, it needs replaced. Don't exclude cracks and dry powder chalking on the exterior. All signs its due for replacement.

 

[TownsendsFJR1300]

+ 1 on both the above posts.

With the age of the bike, rebuild both the caliper and MC as well as replace the brake line (I'd go with SS as its likely cheaper and will perform better than rubber).

The rebuild kit for a single piston caliper is going to be relativly cheap, easy to do and well over due...


The potential consequences of cutting corners in this instance, IMHO, are not worth it..