Author Topic: '76 750F - rear brake still not working! (Read 2148 times)

Tugboat · « **on:** June 29, 2011, 11:35:05 AM »

Replaced my brake lines with ones from @pampadori (they're great btw) and now I have no rear brakes. I've run about 3/4 can of brake fluid through it and am not seeing any more bubbles. The brake pedal pushes all the way down and will stop the rear wheel spinning on the center stand but has basically no effect when you're actually riding it. Also I'm not seeing any leaks. I even tied a brick to it to hold it down overnight as suggested by @silverback - no change.

Anyone know what could cause this? And can I fix it without spending $40 on a MC rebuild kit and/or replacing the caliper?

VonYinzer · « **Reply #1 on:** June 29, 2011, 11:38:19 AM »

If it was your caliper, you woule PROBOBLY see leaking. Could be a bad master. The seals could be shot, and not creating the proper pressure in the system.

May also be that the pads themselves are bad. How old are they? Within spec? Did you at any time get a bunch of brake fluid on them or the rotor?

Tugboat · « **Reply #2 on:** June 29, 2011, 12:00:29 PM »

Pads look okay. Not new but not all used up by any means. No brake fluid on them or the rotor. The pedal pushes all the way down, and when it does you can hear fluid moving about inside the MC but there is no buildup of pressure at all. In other words, it seems like there was enough pressure to push fluid out the bleeder valve but not enough to make the pads grab the rotor? Does that make sense?

lone*X · « **Reply #3 on:** June 29, 2011, 12:14:24 PM »

And you had working rear brakes before you changed the hose? If you did then you need to be looking at the work done when the hose was changed or the hose itself. Changing the hose in itself shouldn't have made the mc or caliper go bad. Are you sure your not pulling air in thru a bad connection?

Tugboat · « **Reply #4 on:** June 29, 2011, 12:53:46 PM »

Well, that wasn't the only change. My bleeder nipple had broken off so after doing the whole "heat the caliper with a heat gun and then shoot the broken screw with canned air and try to back it out" trick that didn't work, I sent the caliper off to Speed Bleeders who drilled out the old screw and installed one of their bleeders for $15 (incl shipping!!). Is it possible that I messed up the seals in the caliper when I heated it up? I didn't think it would since they get pretty hot under braking conditions; and I figured that if that's what was the problem that I'd see fluid leaking out?

jdubb1977 · « **Reply #5 on:** June 29, 2011, 01:04:59 PM »

i wouldn't think that you messed up the seal in the caliper. if you had, you'd be leaking fluid. in fact, anything that would leak air at/near the caliper would be leaking fluid as well. i'd probably start by checking the master. did you take it apart or clean it? could you have damaged anything in there?

Tugboat · « **Reply #6 on:** June 29, 2011, 01:17:16 PM »

I wiped it out but that's about it. It was pretty nasty - fluid probably hadn't been changed in forever. Looks like I'm in for a rebuild then..... Bummer. Those kits are f$#king expensive!!

BeSeeingYou · « **Reply #7 on:** June 29, 2011, 02:07:41 PM »

I would go ahead and get the MC rebuild kit as in my experience when you take an old one apart to clean often it will not seal or function properly when put back together. I think I payed around $40 for an OEM kit two years ago.

scottly · « **Reply #8 on:** June 29, 2011, 07:32:09 PM »

This sounds like another case of air in the system. If the MC is not leaking fluid, it shouldn't need a rebuild kit.

Spanner 1 · « **Reply #9 on:** June 29, 2011, 07:59:55 PM »

Hey Tug... if your getting solid fluid out the bleed nipple, then the seal in front of the piston in the mastercyl. is bad IMO and allows fluid to shoot past it when the brake is applied. If the seal edge is 'torn' it will only make a small pressure on the brake pads and would stop the wheel turning with no 'load'. The bore of the m/c 'prolly is corroded which would tear the edge of the seal making it a non seal

Also an old seal can fail suddenly and lose it's rigidity ( tell me about it

) to where it flexes and loses the required seal.

scottly · « **Reply #10 on:** June 29, 2011, 08:57:21 PM »

It was working before draining all fluid to change the hose. There is air trapped in the system; it only takes about 1/2 CC of air or less to cause the symptoms described.

Spanner 1 · « **Reply #11 on:** June 29, 2011, 09:08:48 PM »

With respect... didn't change any hose, only repaired the caliper bleed nipple and ran 3/4 of a can of brake fluid thru' it ( 1 short hose, compared to the front brake ) and saw bubbles first then solid fluid ...... inverted seal when pressure applied... have had an experience of that happening, and suddenly ! is still M.O.

scottly · « **Reply #12 on:** June 29, 2011, 09:21:07 PM »

Quote from: Tugboat on June 29, 2011, 11:35:05 AM

Replaced my brake lines with ones from @pampadori (they're great btw) and now I have no rear brakes.

Anyone know what could cause this? And can I fix it without spending $40 on a MC rebuild kit and/or replacing the caliper?

Spanner 1 · « **Reply #13 on:** June 29, 2011, 09:32:56 PM »

Duh !... if I had bothered to read back to post#1 then Yes, new brake line... it's still such a 'short' system m/c, hose/ caliper it's hard to believe it's air still ..... bad m/c seal is my bet !

scottly · « **Reply #14 on:** June 29, 2011, 09:38:58 PM »

I bet you a cyber beer that there is still air in the system, and proper bleeding procedure will fix it!

Spanner 1 · « **Reply #15 on:** June 29, 2011, 09:42:12 PM »

Yer on !... I trust Tugboat to make a fair adjudication .....

scottly · « **Reply #16 on:** June 29, 2011, 09:56:14 PM »

I also believe Tugboat will be fair and honest. If I'm not mistaken, he is also a fellow "bearer of the lizard", and can be trusted.

BeSeeingYou · « **Reply #17 on:** June 29, 2011, 09:56:31 PM »

I'm in. I am under the impression Tug took the MC apart for cleaning and like I said sometimes when the seals are old and hard it won't work again. Now it might leak or maybe not, if the primary cup is bad no fluid will escape. The primary cup should also have the cup side facing the spring. Just checking, not implying.

Also as the MC is lower than the caliper I am not sure holding down the lever overnight will help like it does on the front MC.

scottly · « **Reply #18 on:** June 29, 2011, 10:00:27 PM »

I think Tug only cleaned out the reservoir of the MC, and didn't remove the piston at all.. If I'm wrong, well, just glad cyber beers are so cheap!

Spanner 1 · « **Reply #19 on:** June 29, 2011, 10:07:16 PM »

Yep, don't think he opened the m/c at all... so seal / bore condition is anyones guess .... yum, cyberbeers !

BeSeeingYou · « **Reply #20 on:** June 29, 2011, 10:07:57 PM »

Where's Tug? I think we are all getting thirsty.

Spanner 1 · « **Reply #21 on:** June 29, 2011, 10:11:38 PM »

' Prolly still at the Bar, chattin'up young ladies..... might be tomorrow before we know

BeSeeingYou · « **Reply #22 on:** June 29, 2011, 11:23:46 PM »

Quote from: scottly on June 29, 2011, 10:00:27 PM

I think Tug only cleaned out the reservoir of the MC, and didn't remove the piston at all.. If I'm wrong, well, just glad cyber beers are so cheap!

You may be right there but since the rear MC is easy to take apart best to do it. If the reservoir was full of junk the bore will be too.

Tugboat · « **Reply #23 on:** June 30, 2011, 07:13:19 AM »

Ha!! You guys kill me!

So yeah, to settle the immediate questions: I only wiped out the MC bowl. Didn't take any of it apart. But like srust58 said, I do know that sometimes it's the old, rusty, crappy fluid in there that makes it work, and once you run new fluid through the system... well, maybe that little chunk of whatever worked its way loose and now it doesn't seal. Plausible?

Also yes - the MC is about level with the caliper.. maybe slightly lower. It could be just air trapped in the system but what, do I have to hang the damn thing from the ceiling to get it vertical enough?!?

Here's the best pic I could find of how it's oriented:

My bet is with Spanner, unfortunately. These MC's are only on the 75 & 76 F's, so hopefully the bore isn't too F'd

And $40 for a rebuild kit?!? WTF?!?

scottly · « **Reply #24 on:** June 30, 2011, 09:15:13 AM »

Tug, can you post a pic of the caliper, looking at it from the side, so I can see the orientation of the bleeder?

BeSeeingYou · « **Reply #25 on:** June 30, 2011, 11:19:13 PM »

Tug
When you go to clean it out the return hole is all but impossible to see looking in the bore. Removing the bolt low down on the outside of the reservoir allows access to the "send and return" holes.

If you get the OEM kit it should include the boot and pushrod (mine did).

I do believe the MC from a 75-8 Wing will work with a very minor modification. It is identical in all but a small detail. My guess is you will be fine after cleaning and new seals.

Radam · « **Reply #26 on:** July 01, 2011, 06:24:42 AM »

I had a hard time getting my 78F to bleed all the bubbles out. I'd go ride a couple blocks then come back and get more bubbles out. After I hooked up a large syringe to suck the fluid through, I finally got it right. This was with a freshly rebuilt MC. I have an extra rebuild kit for a 78F(rear). If it will work for you I'd sell for $25.

Tugboat · « **Reply #27 on:** July 01, 2011, 10:43:44 AM »

Don't have a pic handy, but see that part on the caliper where the paint has come off? The round part that the piston sits behind? It's right above that and is parallel to the orientation of the caliper.

BeSeeingYou · « **Reply #28 on:** July 01, 2011, 10:51:52 AM »

here is a pic of mine.

Tugboat · « **Reply #29 on:** July 01, 2011, 11:11:21 AM »

Thanks Adam - nope, different part numbers. 43510-392-305 is the OEM rebuild kit for 75-76 and 43530-410-305 is the kit of the 77-78. Guess I'll order one this weekend. I've been working at a shop lately and can probs get one at wholesale cost.

scottly · « **Reply #30 on:** July 01, 2011, 11:25:42 AM »

The point where the bleeder hole goes into the piston bore has to be the highest point on the caliper. I have an after-market caliper, and have to rotate the caliper on the axle to get the bleed passage in the right position, as well as have the bike leaning to the left. It looks like the F caliper has the bleeder off-set from the center of the bore to compensate for the rearward tilt of the caliper, but it may help to raise the rear a bit? (without seeing the orientation on the bike, it's just an educated guess...)

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Author Topic: '76 750F - rear brake still not working! (Read 2148 times)