Rear Brake Won't Bleed

[postmanpat]

We've a 2000 mod 250TXT,the prob started with no rear brake pressure,we tried bleeding from the caliper backwards with a syringe,still no good,we can get a bit if we pressurise from reservoir end.Help!!!!!!

[mountaingoat]

We've a 2000 mod 250TXT,the prob started with no rear brake pressure,we tried bleeding from the caliper backwards with a syringe,still no good,we can get a bit if we pressurise from reservoir end.Help!!!!!!

Hi,have you made sure that there is a little free play on the brake rod that pushes the piston in your rear brake master cylinder (this adjustment is important as it ensures that the piston is returning fully thus uncovering the port that allows fluid in to the cylinder and sometimes you have to back flush the system by pushing back the pistons into the caliper which will displace the fluid (and air ) back into the reservoir.

[graeme17]

We've a 2000 mod 250TXT,the prob started with no rear brake pressure,we tried bleeding from the caliper backwards with a syringe,still no good,we can get a bit if we pressurise from reservoir end.Help!!!!!!

Hi,have you made sure that there is a little free play on the brake rod that pushes the piston in your rear brake master cylinder (this adjustment is important as it ensures that the piston is returning fully thus uncovering the port that allows fluid in to the cylinder and sometimes you have to back flush the system by pushing back the pistons into the caliper which will displace the fluid (and air ) back into the reservoir.

Also check the banjo bolts are tight.

[jse]

"We've a 2000 mod 250TXT,the prob started with no rear brake pressure,we tried bleeding from the caliper backwards with a syringe,still no good,we can get a bit if we pressurise from reservoir end.Help!!!!!! "

Rear brake bleeding can drive one bonkers (which for me, my friends tell me, would be a short trip) but maybe I can give you a couple of tips that might help.

If the rubber boot on the master cylinder is cracked or coming off, there is more than a good possibility that fine grit has entered the piston area (grit + fluid=grinding compound) and worn the seal to the point that it does not have sufficient initial lip pressure against the M/C bore walls to fully pressurize the line. This is common with clutch M/C's and usually the symptom is the lever travels about 1/3rd of the distance before pressure starts to build in the system, in spite of there being no air in the line. This is due to normal wear at the front of the bore resulting in a few thousands taper.

There may be an air bubble trapped in a loop somewhere, under fittings is common. Sometimes pressurizing the system amd cracking the fitting can allow air to escape will work (especially the Hebo 4-puck systems with the spacer under the tall fitting). Trials brake system M/C's displace a very small amount of fluid, so pressurizing them by pulling the lever is sometimes inefficient, but taking the lever off and using a #2 Phillips to push the piston the full distance of the bore works better, especially on the rear brake. Although I personally havent tried it, some riders have told me that hanging the bike up by the rear wheel overnight allows any air in the system to travel to the highest point (the rear caliper) where it can be later bled off.

The advice about having a little clearance between the lever plunger and the M/C piston is spot on and one of the most common problems I see in Trials hydraulic systems (brakes and clutch). The piston must fully retract back to it's stop at the circlip or the bleed hole between the reservoir and the piston bore will not be opened and the system will be "closed" and unable to equalize itself, which can result in some nasty problems. At a Trial I rode in the Northern California Sierra Nevada mountains a few years ago, there was a very long, steep downhill part of the loop that lead back to the pits. Several riders had adjusted their rear brake levers down too far (without shortening the plunger rod to provide clearance), which pushed the piston past the bleed hole, closing the system. When the disc started to heat up at the top part of the hill, heat was transfered to the fluid, which expanded, pressurizing the system further, adding more pressure to the disc, causing more heat to the fluid etc. When they finally slid to a stop at the bottom of the hill, smoke was coming off the caliper and the disc had a dull red glow and now suitable only as an expensive Frisby. The caliper bores had also warped and the complete caliper had to be replaced.

Hope this helps.

Cheers.

Jon

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