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Pesky rear brake...can't bleed #147738 08/31/08 8:51 pm
Joined: Sep 2007
Posts: 504
Sisyphus Offline OP
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Posts: 504
I upgraded to the stainless master cylinder on my T140 and ever since then the rear brake feels spongy to me. Seems like more travel in the pedal than before.
I've tried to bleed it a few times now, the last attempt yesterday with the caliper off the wheel and suspended so the bleed nipple was higher than the rest of the assembly. I get air coming into the line I use to bleed the brakes (hooked to a vacuum gun) but it seems to be coming in around the threads of the nipple. I used thread sealing tape once, I then tried to pack oil-based modeling clay around the base of the nipple to preclude air getting in that way but still I get little bubbles coming up and the brake still feels spongy. There aren't any leaks in the system.
Is there a trick to doing this successfully?


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Re: Pesky rear brake...can't bleed #147739 08/31/08 9:20 pm
Joined: Aug 2007
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Blapper Offline
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Hi Sys,

During the time before I did my brakes I trawled for hints and got some (like disconnect the rear caliper and let it hang and turn the caliper to get the fluid into the rear cylinder). But I found one that seemed interesting in particular. It was drilling through an old bleed nipple and back-filling the system.

During the trawl I found the story of one guy who was already into his second bottle of DOT5 and couldn't get the air out of the system. Now when I bought all the parts to do the job, I had only bought one container of fluid to fill both brakes so was a little concerned about the possibility of a problem causing me to run out of fluid. What I did was this (rear brake described as it is the more difficult):

Drilled through the old nipple and used a length of small bore clear pipe to connect a large syringe full of fluid to the nipple. HINT; you may be able to buy your syringe cheaper at the vets than other places, plus they sell HUGE ones.
Start to fill the system and make sure you stop after about 20ml to move the rear caliper around so you help the fluid into, and air out of the rear cylinder, then carry on until your reservoir is full. A couple of minutes spent being sure of this is time well spent because there is no air purging of the rear cylinder when you bleed.

Now comes the clever bit: pump the pedal down quickly and release it slowly to force any residual air out of the master cylinder and or caliper until you have nearly emptied the reservoir or until the syringe is fully extended - whichever comes first. Now, a$$uming you haven't got loads of shyte coming out of your system (i.e. have done a thorough overhaul and clean), you hold the syringe handle upwards so that the air is at the top and force the fluid back through the nipple into the system again, stopping short of the air of course). You shouldn't need to repeat the process, but you can.

The only little tweak for the front that I have found is that with US bars, that last degree of hardness of the lever is gained by leaning the bike right down to the left and turning the bars to the left to get the last air bubble out of the area where the hose joins to the master cylinder which is the highest point in the front system.

This system is good for other DOTS too even if you don't pump the fluid back into the system because there is less chance of damaging brake fluid getting onto your paint.

IF you see all sorts of crap coming out, you can discard that syringe full and refill with clean before starting again.

If you're using DOT 5 note that DOT5 affects the particular rubber of the disposable syringes so you want to do this in one session then throw away the syringe because it gets too ratchety to use the next day.

I filled and bled both brakes in minutes and only used half a bottle of fluid!

Hope this helps!

Blapper redwine

Re: Pesky rear brake...can't bleed #147740 08/31/08 9:49 pm
Joined: Sep 2005
Posts: 226
Melbourne Metisse Offline
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Posts: 226
The first question you have to ask is 'how old is the flexible brake hose?' Hinckley recommend replacement of flexible hoses every 4 years!

Old hoses decay from the inside out so may look fine from a cursory glance. The carp that flakes off from the inside of old hoses can partially block the hose, bung up bleed nipples, act like one way valves etc. Believe me I've seen them all, including hoses on Bonnies that were over 25 years old!

A new hose is cheap peace of mind.

Re: Pesky rear brake...can't bleed #147741 08/31/08 10:05 pm
Joined: Mar 2005
Posts: 10,329
J
John Healy Online Content
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J
Joined: Mar 2005
Posts: 10,329
"Old hoses decay from the inside out so may look fine from a cursory glance."

Amen
j


Re: Pesky rear brake...can't bleed #147742 08/31/08 10:25 pm
Joined: Sep 2005
Posts: 1,952
T140V-Rich Offline
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I just did Blap's method about six weeks ago. However, I found a cheap (and large) syringe in the automotive/boating section at a larger Wally World. I think its original intent was to measure 2-cycle oil for mixture.

With a bit of 1/4-inch fuel, I pumped fluid in at the nipple and filled it to the level I wanted in the reservoir.

After a decade of so-so rear brakes, I have now have tire sliders, if pressed hard enough.

As Met said, though, I'd replaced the top and bottom hoses.

Richard


1977 T-140V
1973 T-140V
2011 Bonneville SE
Author of "Relics and Reminiscing."
Re: Pesky rear brake...can't bleed #147743 08/31/08 11:10 pm
Joined: Aug 2005
Posts: 1,208
S
Steve in Tulsa Offline
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Quote
I used thread sealing tape once, I then tried to pack oil-based modeling clay around the base of the nipple to preclude air getting in that way but still I get little bubbles coming up and the brake still feels spongy.
First, the wrong tape is destroyed by brake fluid. I don't know which one you used.
Second, I have the same feeling about modeling clay.
I have the feeling you have the pedal adjustment too far out. The piston has to be pushed in just past the bleed back hole to actuate the caliper. Not covering the hole completely will give this feeling in the brakes. Try turning the cylinder into the housing one turn and then test.
Just don't test at high speed yet.
HTH


Steve in Tulsa

Moderated by  John Healy 

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