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'87 R80RT brakes - Mighty Vac doesn't suck?

RANBUSH

RANBUSH

Ran Bush
Apologies if I posted this already, if I did I can't find it.

The front brakes on my '87 R80RT locked up the other day a short way from home, and the bike wouldn't roll. I loosened the caliper bolts slightly and that freed the front wheel enough to get home. When I got it home I removed the OEM brake lines, cut them in half, and saw that they were plugged up and rotted.

So I installed SS lines but when I tried to bleed the system today I got no pressure at the lever. I was using a new Mighty Vac vacuum pump and it drew some 2" of fluid out of the lines, and the fluid in the reservoir went down to the Minimum mark but when I tightened the bleed screw, removed the Mighty Vac and pumped the brake lever, I got no pressure at the lever and the fluid in the lines had returned to the Maximum mark in the master cylinder reservoir.

I tested the Mighty Vac on a jar of the used brake fluid, and it sucked it up and seemed to be working as expected.

I'm thinking the master cylinder may also be shot.

Ideas ?
 
This happens often with brand new lines. I take it yours has the MC and reservoir on the handlebar. Here are the standard tips -- The first thing I would do is try the old method of bleeding -- hose into bottle of fluid, pump the lever, open the bleeder, close, repeat. You can also try this with the Vac attached. The next is to get a syringe and back bleed it.

The puzzling thing is the fluid pulling back into the reservoir. It is possible for the passageway between the MC and the reservoir to plug up, usually from extremely old fluid or is someone mixed DOT 5 and 3/4. IIRC your MC is integral with the reservoir. With clean fluid in it you should be able to see (with the cover off) the passage open and close when you operated the lever.

The MC may need rebuilding. I'd certainly think about doing so on this old a bike, particularly if I didn't know its service history. That said, calipers suffer from old fluid before the MC does.
 
You could also try leaving the bike on the side stand with the front wheel turned all the way to the left.


Cover off the MC and squeeze the brake lever..keep repeating and see if any air bubbles pop out. Also, check all the connections for any leaks.
 
Is it possible to have a stuck piston which when vacuum is applied sucks in but when suction released it moves back to its original position causing the fluid to rise? I'm not at all familiar with the brakes on the air head.
 
Ditch the vacuum tool.
It’s common to have to bleed the master cylinder when hoses and fluid are completely removed. Master cylinders from that vintage don’t have their own bleeder port like many of the newer ones, so there are a couple of things you could try. First, protect all nearby surfaces with damp rags then crack loose the brake line at the MC and gently work the lever until you can consistently push fluid at that point. If you can’t get it to that point, retighten the line and with fluid in the reservoir turn the bars full left and tie the brake lever back almost to the bar, as was mentioned in earlier post. Leave the bike overnight then try conventional bleeding again.

I’ve never been a fan of the vacuum tools—they rarely seem to seal well and can cause unintended effects on the internal rubber bits, including OEM rubber hoses. That’s especially true on post-airhead bikes with ABS.

Best,
DeVern
 
GTRider said:
Ditch the vacuum tool.
Master cylinders from that vintage don’t have their own bleeder port like many of the newer ones, so there are a couple of things you could try. First, protect all nearby surfaces with damp rags then crack loose the brake line at the MC and gently work the lever until you can consistently push fluid at that point. If you can’t get it to that point, retighten the line and with fluid in the reservoir turn the bars full left and tie the brake lever back almost to the bar, as was mentioned in earlier post. Leave the bike overnight then try conventional bleeding again.

DeVern
Click to expand...

Ditto. As to the fluid being drawn back into the reservoir -- did you have the cap off? Is it possible the vent to the air in the reservoir cap and cover is blocked?
 
ranbush said:
Apologies if I posted this already, if I did I can't find it.

The front brakes on my '87 R80RT locked up the other day a short way from home, and the bike wouldn't roll. I loosened the caliper bolts slightly and that freed the front wheel enough to get home.

SNIP

I'm thinking the master cylinder may also be shot.

Ideas ?
Click to expand...

I’ve worked this issue on a couple of different bikes. The master cylinders I’ve worked on (BMWs and other brands) always had TWO holes, one small one rather easily seen at the bottom of the reservoir and another truly tiny one a bit closer to the caliper line output, which is often very hard to locate. The tiny forward hole allows fluid pressure in the system to be released when the brake lever is at the rest position and the piston is fully retracted. If the tiny hole gets plugged, friction heat from the caliper will build pressure in the brake system and auto-apply the brakes. Locating and cleaning the tiny forward hole in the master cylinder fixed the pressure build problem every time.

There may be other master cylinder designs without the tiny hole, but I’ve never worked on one.

Good luck with your repair.
 
Tiny hole is in the right side depression. If plugged, will cause the brake lock problem.
 

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Thanks to all for the replies, I will try eliminating the Mighty Vac from the situation and use the old-school bleeder method.

If that doesn't fix it, I also found a MC rebuild kit in my spares bin and I will try installing a new piston, rubber and spring to see if that makes a difference.

Thanks again.
 
Happy ending today; per Internet Wisdom, I pulled the bleed screw, sprayed it with brake cleaner, wiped it a few times, and wrapped the threads with teflon tape. That was enough to make the Mity Vac happy, and now I have good front brake performance.

I also found some good (but old) reviews here about Speed Bleeders and I checked out his website. Seems like he's still making them, anyone have any recent experience with the product?
 
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