Hydraulic handbrakes: myths, truths and how-to guide

Posted: May 7, 2015 by Ben in How-to's
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Bleeding a hydraulic handbrake can either be painless, or a source of never ending problems. In order to help others I’ve decided to post up my learnings.

I’ve had a lot of problems with mine. I went with a full PSM kit hoping to avoid some of the other issues but the bleeding has been horrendous.

I’ve seen some of the advice given online and a lot of it is bogus. Here are some hard and fast truths if your handbrake does not lock the rear wheels.

(HMC = Handbrake master cylinder)

-Rekit your rear callipers if you can – fresh seals will give the pistons the best chance of moving freely
-Bench bleeding the HMC is a must. Ensure all your HMC fittings are attached (adaptors, 90degrees, etc) and that the bleeding hoses are secured tightly on the fittings via clamps. You can detect leaks easily during this process as you will hear them. I’ll do a how-to below.
-Do not overtighten fittings – remember the threads are small and not very long. If during the bench bleeding you see fluid seeping from a fitting, or can hear bubbles entering, then tighten fitting gradually.
-Foot bleed the HMC feed line (from the BMC) into a jar in the car before fitting it to the HMC. Ensure the end of the line is submerged in some brake fluid to avoid it sucking air back in as the pedal comes up.
-Once all lines and the HMC are connected, bleed the output line from the HMC to the factory rear brake line using only the foot brake.
-Start bleeding the full system on the foot brake and do not touch the handbrake. Once you’ve bled both rear callipers, bleed both using the handbrake. This will help any air trapped in the HMC to be pushed out.
-If the HMC goes straight to the bottom you either have lots of air in the lines (unlikely) or the lines are around the wrong way (assuming you have the cap off the BMC reservoir). If you have some resistance then they are plumbed correctly and just require more bleeding. Check fittings for leaks.
-After tightening up the bleed screw after bleeding a calliper, do not pull the handbrake back up, pump it up using the footbrake. Forcing fluid into the HMC will push air out of the MC better than the plunger drawing in fresh fluid.

And most importantly…

-Check your fluid after every 3rd bleed. The amount of fluid the system can move can be extreme.

Bench bleeding a Handbrake Master Cylinder:
Traditional bench bleeding doesn’t really work due to the lack of a reservoir. But as both ports draw and feed on either the inward or outward stroke using a clear jar as a reservoir means it can still be done.

Step by step guide:

-Put the master cylinder in a vice, ensure it is level
-Cut the top off an empty clear water bottle
-Half fill with brake fluid
-Put rubber lines on the fittings, clamp them up tight
-Put both ends of the lines in the water bottle but ensure they are at least 20mm from the surface. This is to ensure any mildly aerated fluid (which floats to the top) is not sucked into the MC
-Screw the handbrake handle fitting onto the plunger and slide a screwdriver through the holes (may be different for non PSM handbrakes)
-Place both hands on screw driver and slowly work in and out
-There will be a lot of air at the start as the rubber lines expel their air
-Keep going until you do 5 in-and-out strokes with no bubbles. Then do 10 more.
-Leave with rubber lines still submerged in brake fluid for 10mins (go bleed the BMC to HMC feed line)
-Repeat bleeding until you do 5 in-and-out strokes with no bubbles. Then do 10 more.

Hope that helps people out there avoid the issues i’ve had


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