XJ750 Seca Brake Rebuild Issue

So, with Chacal's help, I rebuilt both calipers and the master cylinder. Cleaned out the little blow hole, the whole nine yards. After the rebuild, I had some trouble bleeding the system, so I (seemingly successfully) "reverse bled" with a large syringe. Fluid comes out clean and clear at the top of the metal "S" hose that attaches to the exit of the master cylinder from both sides when pressurized from bottom. I had the master cylinder primed, and it shot out plenty of fluid, however, still no pressure on the system.

I took the master cylinder back off, disassembled it, checked everything and have discovered the following. The master cylinder pressurizes the exit just fine, however it does not pull any fluid from the reservoir. When I push the plunger in with my finger over the port, it will push out air (or fluid once filled manually) past my finger like I would hope, however - on the return stroke it just creates suction. If I poke the tiny blow hole with a wire, it will release the suction, but ONLY if I manually poke the tiny hole. It doesn't seem to be dirty. Is something behind the hole blocking this hole? Should it be? I'm not entirely sure I understand the function of this little "pressure relief" hole other than a return hole for fluid.

Bottom line. I'm out of ideas and will take any ideas that you might have. I'm a semi-competent shadetree mechanic, but have formerly only worked on vehicle of the four-wheeled variety. Throw any ideas you might have at me.

Thanks in advance for your thoughts.

 

The only thing behind the return port to block it will be crud that didn't get cleaned out.
My WAG of the week is that you kept the original brake lines. If I'm right, that's where the crud came from.

 

I agree with K-moe. You probably have a bit of the old rubber brake lines blocking the m/c. By the way, when I reverse bled my completely rebuilt system, including new brake lines, I started with no brake fluid anywhere, including the M/C, and all connections were tight. So I actually pumped fluid into the calipers, brake lines and then into the M/C. If I had had a blockage it would have not worked. Also, stainless brakes lines are a real improvement.

 

Have you checked to make sure the piston is returning all the way . Cable too tight not allowing piston full travel.
If you are still using cable op master cylinder.

 

Thanks for the suggestions. I was traveling for work for a bit, and am now back on this issue. Here's where I'm at now.

Thought that the issue could be the brake lines - great thought - I hadn't changed them. However this cannot be the entire issue, the problem persists even with the mc removed, cleaned out and in my hands. I'm starting to wonder if I don't have an incorrect piston in this thing. It seems as if the reservoir/mc hole (large one) is smack in the middle of the piston when inserted (can see the spiral bare metal through the hole) and the little blow back hole is possibly covered by the smaller seal that sits on the end of the spring - in either event not allowing fluid to move freely between reservoir and mc.

I'm trying to attach a photo and will immediately follow this post with a photo. Can anyone verify if these are the correct parts for a (behind the headlight) 750 Seca? Is this truly how the system should work (with the piston resting over the hole at full extension/resting on the retaining ring)? Image approximates where the internals rest when installed.

As is, the mc will not pull fluid from the reservoir even when operated in hand.

Thanks again for your help.

 

Those look to be the correct size/type items (and, they are oriented correctly).

Specs:

Piston
OL: 25.45mm
OD: 17.40mm

Spring:
capped
OL: 49.60mm
OD1: 16.85mm
OD2: 11.50mm
Wire: 1.36mm

 

WHen I rebuilt my MC, I insisted a bit too much when I sanded the inside of the barrel the plunger travels in, The result was that I wasn't able to build any good pressure in the system with the old rebuilt MC. I finally bought a new MC and pressure built up within seconds. JUst sharing this, I don't say you're experiencing the same.

 

Have you tried blowing out everything with high pressure air nozzle? I still suspect some crud or bit of rubber from the brake line is causing the problem.

 

If the interior of the brake lines has decomposed......and they do decompose....

However.... It won't push fluid when held in hand. The relationship between the piston, seal, and holes that you describe is correct. Have you inspected the bore for nicks or bits of hardened glycol that could be disrupting the seals?

 

It's not a matter of building pressure. When pressed, the system builds pressure and fluid moves from the cylinder as it should out of the cylinder exit. The seals seat properly, do not leak, and return well. The problem is that the cylinder does not recharge with fluid from the reservoir. I've gone as far as to remove the reservoir with seal and fill the seal area with brake fluid and operate the cylinder. The cylinder moves as normal and pushes fluid out the exit as it should. However, if I put my finger over the exit, allow the piston to return and press again, the MC does not pull any fluid from the reservoir. Does that make sense?

 

The "recharge" should be coming from the brake lines as you release the lever. What's in the resivour gets uset to fill up the space when the caliper pistons move. When you release the lever the caliper pistons retract, and the excess fluid returns to the resivour through the (very tiny) return hole.
What you describe is normal since you aren't working with a closed system at that point, and any fluid that is lost can't return.

 

The bottom of the piston seemed to be a little crudded up, so I sanded it back down and all seemed to be well. The piston returned to c-clip and pulled fluid from the reservoir. HOWEVER, upon reinstallation and all of the bleeding, the piston does not willingly return the full length to reseat itself. It stays seated about half way up the cylinder and only very slowly returns to the bottom. I'm about crazy with this here.

My only thought is that perhaps the spring is toast or the walls of the cylinder need polished? The spring still seems very springy (difficult to compress by hand), but perhaps needs to be even stiffer? I only replaced the seals. As far as the walls of the cylinder, they do not appear pitted or corroded and feel quite smooth. When I have the cylinder removed and run the whole system with soapy water, it is slick as can be and returns immediately. The issue seems to be friction related. I'm assuming that the piston should always snap right back without any resistance. Any ideas? I'm assuming that there is no sort of lubrication that can be used inside the system besides brake fluid.

 

Do you have the original parts to compare side by side?
Also possible that you have nicked the blow hole with the wire cleaning it out and left a burr on it.

 

Had a very similar problem for the past day on my 750 seca- MC piston was locked at the top of the cylinder. Could operate it fine when removed from the bike, but upon install, it lodged itself in the top again.

Removed the MC and tore it part again this afternoon. Used a high e string from a guitar (left over from carb rebuild) to poke the little hole clean. Cleaned MC body with brake fluid, dried it out, then re-finished the bore with some left over sandpaper from anti-dive rebuild kit from Chacal (about fifteen minutes of slow sanding using said paper & a socket extension.) Cleaned it again with brake fluid & shop cloths. It's refilling from the lines/reservoir now; my brake lever gets pulled back out by MC.

Off to track down the air leak in my lines now!

Edit- I prefilled/reverse bled the calipers/lines/splitter/hardline and the MC before reinstall.