Bleeding the Rear Brake

Brad's Method

Having read how difficult I found bleeding the rear brake, Brad Baument was kind enough to share his method with me.  For those who don't know Brad, he was co-founder of RYP with Ryan Young and remains a very busy man with the AMA.  He also competes on an ePure Race.

Brad developed this technique while working on Shercos at national events where multiple bikes must be processed quickly.  He got it down to 30 minutes, which included removing the rear wheel and exhaust system.  Since the EM chassis is basically a Sherco/Scorpa design, the technique is applicable to a variety of trials bikes.  Of course, the EM has no exhaust system with which to contend.

My criticism was that this method requires 4 hands.  Brad countered by saying the caliper and master cylinder must be clamped into their new positions.  I shot two photos of this for illustration.  The technique boils down to the following:

1. Dismount the master cylinder and get it high.  The reservoir must also be dismounted. 

2. Dismount the caliper and get it low.  Insert something to substitute for the disk.

3. Use a tool to push on the master piston.  It's imperative to get the full stroke and allow the piston to return completely.

4. Bleed as you normally would from the nipple.

Eventually, I achieved success with this method but still experienced a difficulty I attributed to my addition of a pressure transducer (which can be seen in the header photo).  A lot of twisting and contorting of the caliper was needed in an attempt to dislodge air trapped in the transducer and fitting. 

I have no doubt the method works well without the added complication of a pressure transducer.

Caliper shown clamped in a low position.  

Master cylinder shown clamped in a high position. The push tool is a long T-handle 4mm Allen wrench.

Alternate Bleeding Method

Having popped my pressure transducer (discussed in the Regen Experiments section) and replaced it with the OE banjo bolt, I wanted to get the rear brake operational again quickly.  I took what I learned in the prior bleeding attempts and came up with the following (it has some advantages and disadvantages over Brad's method): 

1. Loosen the brake pedal adjustment and height limit screw to allow free play.  This comes from Brad's method to make certain the master cylinder piston is free to fully return.   But you can now leave the master cylinder in the stock location.  I suspect Brad did not want to do this to competitors' bikes at events because everyone has their pedal set differently.

2. Completely remove the bleeder screw and wrap a few turns of Teflon tape around the threads.  Re-install.  This prevents air from leaking in around the threads.  I plugged the caliper hole while wrapping the tape as fluid will leak out.

3. Make a siphon bottle as shown in the above photo.  I originally took that photo to show Brad's technique of getting the caliper as low as possible.  But that's not necessary - neither is removing the rear wheel.

4. Fill the bleed hose with brake fluid and install it on the bleeder nipple.  Open the nipple slightly and set the waste fluid bottle on the ground.  A gravity siphon will start.  You don't need to worry about closing the bleeder nipple because of the siphon and Teflon tape.  Now just pump the rear pedal until all the air is expelled.

5. Make sure the reservoir is topped up before you begin.  You may need to refill the reservoir several times depending on how much air is in the system.  Because I had replaced the banjo bolt, mine took 4 fillings.  I also tapped on the hose and caliper with a cushioned tool to help dislodge any bubbles.