Rear Brake Bleeding

[kevin j]

The most discussed curse of the entire trials bike: bleeding rear brakes.

Actually it was a bit spongy, so I was flushing fresh fluid through, then trying to bleed it out better.

96 GasGas. The actuator is at the bottom, so the highest point on the master cylinder bore is above the inlet, and above the outlet.

Done all the basics that are covered on every post: removed pads, shimmed pistons apart, syringe pressure from the caliper end. Pushed through as fast as possible. Piston bleed port port is uncovered. Work the pedal very small strokes.

I am careful to not overstroke the piston (Ishy or Ringo's tip about drawing air in the piston seals on return stroke.) I would try vacuum from the reservoir end, but I think they are right, I'd just pull more air into the piston seals.

I unbolted master cylinder, contorted it to get the inlets/outlets to the highest point. Tilted the caliper down to be the lowest.

Tapped, vibrated, and wiggled things more than a middle eastern dancer.

Still some air in there. There are brakes, but the lever bottoms on the frame before locking the wheel.

Any other ideas?

k

[brucej]

Kevin, you could try using a oil pump can instead of a syringe to pump the fluid in. I don't remember for sure where I got this idea, but I think it was from Ron Milam's tips. It does work better than the syringe, at least for me, because the pump is easier to work with one hand and it seems to develop more pressure.

A couple of other problems I ran into when I did this job on my 98 GG yesterday: the bolts that hold the caliper together weren't tight, so it was sucking air. I've never had the caliper apart, so they probably just vibrated loose over time. The other problem was in getting the banjo connection to seal at the caliper. I did have the line off the caliper, and the sealing washers had 'seated' in their original position which, of course, I couldn't replicate. I'm sure new washers would be the best fix for this, but I didn't have any, so I filed the old ones flat and it seemed to work.

I don't know if any of this will be of any use to you, it did work for me but maybe that was just luck. Speaking of which, I hope yours gets better!

[windlestone]

I have written a saga on my rear brakes in the gas gas pages. It took me 7 days and nights if you include thinking about it every waking minute. It appears to be one of those problems that either works first time or it chooses when to work.

I dismantled swinging arm to allow master cylinder and reservoir to come through and then i suspended my bike by the front wheel from the garage ceiling beams.Hold the reservoir at the highest point and the master cylinder next highest.Completely empty system then fill through rear brake caliper bleed screw with a syringe. When the fluid enters the reservoir maintain pressure with syringe and empty some fluid from reservoir and continue to force fluid through. I found i was pushing air in through rear bleed nipple where the hose was connected so make sure yu have a good tight fit. The problem area is the hose between the rear caliper and the master cylinder where it comes through swing arm. When i had reservoir filled i tapped reservoir hose and bubbles came up through reservoir. When they stopped appearing i bled system from master cylinder then caliper brake hose then caliper bleed screw. This is not a quick process and i probably bled each place about 20 times. I then tapped all hoses again and watched air bubbles coming up through reservoir. VERY FRUSTRATING. I then continued to bleed in the same sequence and i started to get a bit of pedal but like you i hit the frame before i had a decent brake. I then cracked open the brake hose at the master cylinder and found i had the most success by bleeding pushing the caliper pistons back then tighten hose off then start again. After what seemed like hours the air finally stopped and i got a decent brake.

Good luck

[windlestone]

i hope i explained that ok. i had the hose in the swing arm as straight as possible thats why i partly dismantled swinging arm then suspended bike from front wheel. i also bought new bleed nipple and washers for all hoses my bleed nipple took some abuse from all the opening and closing required.

have caliper swinging free on the end of the hose and move hose in and out of swing arm as much as it will allow.

Good luck again and let me know what finally cures it please

[j@mes]

don't know if this is a particular problem that affects gassers but one i bled my back beta brake all i did was loosen the valve fit a tight pipe to the valve pumped the peddle and just kept topping up the resorvior after second attempt it worked and i not a very competant mechanic with not a lot of experience

[dr nick]

The one and only time I tried to do the back brake on my Techno it took days and days of trying different ways (system on and off bike).

When I used the syringe method , I used a vetenary syringe first (about 50ml) but that cracked under pressure after a while(a bit like I did doing the job)so had to get another ,but I could only get hold of a small one (10ml max) I found that the syringe was what was causing lots of air bubbles ,every squeeze of the syringe, lots of microscopic bubbles,I ended up with a brake ,but like windlestone,it kept bottoming out on the frame.

I got a mechanic to do it for me in the end...best

[alan bechard]

I use the big syringe.

Try pushing from the resovoir as well as from the caliper. the nose of my syringe fits snugly in the hole in the bottom of the resovoirs.

Insure that you can push both ways freely with syringe. Check your adjustment. The little bitty hole that you are pushing through in the master is really little bitty. The smallest chunk of junk can block it.

I had problems with the elbow (not sure if yours has one.) that connects the resovoir line to the master was not truly round. when you rotated it a bit it would break the seal and allow air in the master.

Patience, and trying different techniques.

Chris UK swore by a vacuume bleeder that is available here from NAPA and from Parts Unlimited Dealers. Retail on it is about $200 so I have not coughed up the money myself yet, but it sure is tempting when things are not going well.

Good luck.

[windlestone]

Just read Alans tips and i remembered the adjustment at the rod/pedal going into mastercylinder is crucial. You must have free play to allow fluid through

[jtt]

I had some success recently using a syringe by not only pushing fluid through, but alternatively sucking it out. Seemed this released some bubbles that just didn't seem to want to simply go one way. Be very carefull though as it doesn't take much to suck the reservoir dry and draw in more air than ever.

[steve]

So you guys have trouble eh, what if you had to bleed hundreds of bikes?

Well this is the sort of thing they used at the Scorpa factory (see photo)

Where possible the systems were built and bled off the bike first, there was a rack with the master cylinders clipped at the top and hose and slave cylinders dangled below, that way gravity lends a hand I suppose!

This one can be bought from -

http://www.machinemart.co.uk/product.asp?p...47&r=2056&g=107

[kevin j]

tks all. will attack again tonight.

Have free play in the lingkage, replenishing port is open, fluid goes back into the reservoir.

Problem seems to be where the hose from caliper makes bend at the swing arm. The high point is in the cylinder, and the blind end of cylinder is higher than either in or out ports. Hence the disassemble and twisting flat (that didn't work).

and yes, chris uk's magic box has crossed my mind a lot.

I need it going for our first event a week away, and found out am leaving town now for the UK, hope to return the night before the trials. son may ride without much rear brake. daughter is set, and at least my old Yam has front disc but cable in back.

i'd love to stay in uk and see a trials over there, but won't see anything except the inside of an engine room.

k

[boofont]

Chris UK swore by a vacuume bleeder

Ah yes, Chris UK!! I remember him....

I think most of the time bleeding the brakes like James has pointed out works pretty well for all of us. Its pretty much the same way the brakes get bled on a Tornado anyway! The key to it is to take your time and don't rush.

However, there is occasions when this just won't work. I've found, and I've worked with a lot of hydraulic systems, that air can get trapped in area's that are high up in the loop. By this I mean that if you have, for example, a hose that climbs up higher than the exit from the master cylinder before heading to the caliper. The air seems to gather at the high point and even though you push the fluid through it just won't budge. As you push on the peddle the air wil move down towards the caliper. You lock off the bleed nipple and release the brake. As you do this the air just moves back to the high point. Its like trying to get rid of that annoying gatecrasher at a party, all the gentle hints just don't seem to work.

To get rid of these gatecrashers you need to be more forceful, hence the technique (all be it taken to extremes, but hey, it worked!) adopted by windlestone.

[kevin j]

boofont: thats exactly what I was trying to deal with, I am sure the air bubble was up high in the top of master cylinder.

so, today got it on the hoist, pulled all the plastic, but before I started bleeding it did a check of the pedal. Perfect brake. minimal travel, no spongy, predictable and firm. Put the plastic back on and lower the hoist and ponder why.

todays UCTT is to to nothing and the problem has resolved itself.

Why do I feel its going to reappear at the most inopportune time?

I am thankful it appears solved now.

k