Problems with Brake Caliper Pistons
Brake caliper repairs can be an expensive exercise when caliper pistons seize, a problem particularly common on boat trailers due to excessive exposure to corrosive environments. Of course, there are solutions and suggestions for ongoing maintenance, however, first, we must understand the underlying problem.
Hydraulic oil is pumped by an electric over hydraulic actuator to the brake caliper. This hydraulic oil pressure varies from 1000 psi to 1600psi depending on the weight of the trailer and the size of the brake actuator.
During braking, the hydraulic oil activates the caliper piston as it enters the cylinder chamber thereby pushing the piston onto the brake pads which in turn causes friction on the disc rotor.
This friction causes braking. The more pressure applied by the brake controller, the stronger the braking.
Caliper pistons are manufactured of phenolic plastic, aluminium or steel. “Phenolic” refers to a variety of hard plastic that is exceptionally strong and heat resistant. Phenolic pistons reduce heat transfer into the brake fluid, are resistant to corrosion that could cause caliper binding and are lightweight.
Whilst it is true that phenolic pistons resist corrosion, they do become scratched over time and can also be left to sit for extended periods. As a result, the hard plastic material becomes hygroscopic.
A hygroscopic substance is one that readily attracts water from its surroundings, through either absorption or adsorption when its surface is scratched.
Badly jammed pistons are challenging to remove. The piston housing will require honing before a new piston and seal kit is installed.
However, before starting the honing process, it is recommended you perform an inspection on the stainless steel slides. If these slides jam, they are in need of replacement and repair costs begin to rise dramatically.
If you find yourself with jamming slides our best solution is to buy new callipers with stainless pistons. Whilst it is possible to purchase a new replacement slide kit, remove the old slide and clean the bore before installing the new slide kit – the time and cost of doing so, often justifies the purchase of a brand new caliper.
If you do wish to refurbish your existing calipers, keep reading for tips to follow.
Caliper pistons are relatively simple to remove with the correct tools, but it is essential not to mark the piston bore during removal.
Couplemate uses a specially designed air hose which screws into the brass adaptor. With this, 70 psi of air pressure ‘pops’ the piston out of its housing.
If a similar air hose solution does not work, then the phenolic piston must be mechanically broken out of its housing. Take care not to mark the piston housing with tools during the piston breakout.
Piston Housing Explained
The piston housing has 2 two holes
- One hole is the inlet for the hydraulic tube. This inlet is 3/16″ UNF where a brass adaptor is inserted to accept the tube or hose.
- The other hole is a bleed hole. The bleed hole contains a bleed nipple which must always be installed in the higher position than the inlet hose.
Immediately after removing the piston, the bore must be honed to remove the remnants of corrosion and any sharp protrusions that may be left in the bore. The caliper hydraulic bore must be in pristine condition before replacement pistons are installed.
Tip: Remove the bleed valve and the adaptor for cleaning. It has been known for small animals to leave nests inside bleed nipples.
When looking at a jammed Phenolic piston, the outer dust cover looks like a seal; however, there is an inner seal that performs all the hard work. The two rubber rings inserted into the piston body include:
- The lower slot – for a rubber hydraulic ring that seals the piston into its housing.
- The upper ring – for a boot ring. It holds the dust boot into position and is a secondary sealing ring.
Inserting new pistons and seals is certainly a challenge for most servicemen and almost impossible for the handyman.
Couplemate prefers to coat the bore and piston with a silicone-based lubricant before inserting new pistons, as grease is not a suitable lubricant for caliper bores or slides.
Couplemate pistons are made in Australia from 316 Stainless. Commercial fishermen prefer stainless pistons as it dramatically reduces their maintenance and downtime.
The caliper in the earlier photos of this post was stainless steel and had not been serviced since 2013.
Phenolic pistons were replaced with Couplemate stainless pistons, new seals were installed, bores honed, and stainless guides served with silicone grease.
We found half of the stainless guides ceased.
Couplemate provides the following services on Trojan or Al-ko pistons only:
- Supply Stainless Pistons
- Supply and fit stainless pistons to new dacroment or stainless calipers
- Supply and fit stainless pistons to used Trojan or Al-ko calipers
If you wish to have your calipers repaired or refurbished with our experienced team, please send your existing Trojan or Al-ko hydraulic calipers to Couplemate Trailer Parts, 136 Glenora Street, Wynnum. 4178. Queensland.
Please ensure you include instructions of the intended work you require, as well as your contact details. Upon receipt of your package, we shall inspect the job and call you with pricing to complete the refurbishment. If you wish to get an indication of pricing, the link below gives an indication of what you can expect to pay to make your caliper like new again.
Thank you for reading this post. I trust the information contained has been informative and has helped you decide on the best course of action for your trailer brake calipers.