Disc Brake Lag and EOH System PSI vs. Time
I have read on two forums that trailers with disc brakes are almost universally accused of having objectionable disc brake lag or a noticeable delay in trailer brake actuation vs. the tow vehicle. I don’t share that observation but I can easily understand where that comment can come from. I really think it comes from an unmaintained trailer or incorrectly setup rig.
I still consider myself a newbie and my observations are based on one trailer only, but I think I have done a lot of work on my trailer brakes, by my unofficial count; I have bled them about 10 times in about 3 years. Several actuator issues, the AS ActiBrake recall, BTW “passed” inspection but I replaced with Carlisle, Carlisle failure, re-installed old ActiBrake, ActiBrake failed, Carlisle repaired and re-installed, longer brake hoses, reconfiguration of longer brake hoses and in the middle of all of this, new Dexter axles, more brake bleeding, disc brake calipers dragging and overheating, repairing calipers etc. You get the idea and the better half has patiently sat in the seat and operated the controller for bleeding.
My rig is a 2007 Classic Ltd. 30’ slide out. Dexter 5,000 Lb axles, Carlisle actuator and a MaxBrake controller.
I primarily addressed two issues as I did all of this work that I want to focus on.
1. Short brake hoses and the resulting kinking. This has been presented several times with several solutions. I feel that I had a single hot brake from a kink early in my experiences.
2. How do you know when the trailer brakes are completely bled? This is a very important issue and directly relates to brake lag.
When AS built this trailer they used a flexible brake hose with inverted flare female and male ends. They routed the lines in from behind the calipers, came over the top of the caliper and rather sharply “curved” the hose into the calipers. See Hose Routing 1. This also requires a 3/8” female to a male M10 bubble coupling as the Dexter calipers are female M10 bubble. What this caused IMHO was a system that COULD trap air in the inboard caliper casting while it is trying to push fluid down to the outboard caliper housing for bleeding purposes.
I reconfigured the flexible hose to come in from underneath, then transition to a steel line curving back into the caliper, see Hose Routing 2. These pic’s are just mockup shots of the components. I also fabricated a short steel bracket to support the line. See Brake Hose to Line for my installed and running version. The single biggest advantage that I feel that this offered is the ability to loosen the nut on the steel line where it enters the caliper and use this as a bleeding point. This allows independent inboard and outboard caliper bleeding. I think if you tried to do this with the rubber hose, you risk a twist in the hose that you really didn’t want. I also truly believe that the mere flow of fluid out of a bleed screw is NOT a guarantee that the entire systems nooks and crannies are bled and AIR free. Air will hide in the physical high points in a system like this, getting an air bubble to go down and out, not as easy as it seems.
If I was starting over on this hose routing, I’d seriously look at coming in from the front and running the line/hose down the trailing arm somehow just like the rear brake hose would be routed on a motorcycle rear wheel brake, as opposed to coming in from behind like the AS.
In your passenger car or TV, your leg applies ONE stroke to the brake master cylinder, pressure is created and braking occurs. IF you had an air bubble in your car or TV brakes, the observation could be that you have to pump the pedal to create braking of that you have to push the pedal farther than normal to build pressure. But with Electric Over Hydraulic (EOH) you start the command for braking with an electric signal from the controller and a pump starts pumping and tries to create pressure. The difference is that the pump will compress the air bubbles in the EOH system and as soon as the air bubble is compressed, hydraulic pressure starts to build. And it is the creation of hydraulic pressure in the system that starts the braking process. This pressure will not exist until the caliper pistons are touching the brake pads and the pads are physically being compressed against the brake rotors. And the delay in the creation of hydraulic pressure compared to the drivers request for braking that I believe is the big source of disc brake lag.
How do you know when the brakes are bled?
I consider myself fairly handy and creative on mechanical issues, and still learning. So to answer this question I turned to a 1,000 PSI gage. I had to make an adaptor from ¼” MPT to M10 bubble. A small block of steel and what is known as a M10 banjo bolt to attach it to the caliper. Then the proof comes from two sources. First the relaxed, confident feel of my braking while towing and second this video.
Hyd Test - YouTube
It is a simple 5 second shot of the PSI gage ramping up. Unfortunately we do have one variable in this, how much of a request for braking did my better half initiate from the MaxBrake slider control? Sorry. But using video editing software I was able to view a timeline from the audio track showing the audible start of the pump soundtrack vs. the response on the gage.
From the start of the audio it was only about 0.5 second to 200 PSI. and it peaked at about 700 PSI.
In my driving observations, I can report a smooth coordinated TV and EOH braking. No scary brake lag, no brake surges. I like my brakes.
What would I look for on your trailer with EOH?
Hose routing. Contact to frame shows as a rubbed spot on the hose, hose twisted and any kink or evidence of a kink at the caliper fitting.
Actuator wiring connections. A frequent topic on all brake electrical circuits, don’t forget the 7 pin.
Controller, just curious, are you sure it is EOH compatible? Not all are.
Excessive rotor runout. If the rotor has warped and is actually pushing the pistons in farther into their caliper bores than normal, it will take a bit more time to get the pistons to go back out and start the clamping process. Time = lag.
Periodic inspections, with the wheel off if at all possible, check the condition of the PISTON DUST SEALS. Each Dexter caliper has 4 pistons and 4 seals. Any tears, cuts or cracks can allow moisture to get in and start corrosion between the steel pistons and cast iron caliper housings. See Corrosion picture. This corrosion can IMHO and experience can absolutely cause sticky pistons creating dragging brakes and the question from the toll taker on the only toll road in SC, Are your wheels supposed to be smoking? Yup, it happened to us. The brakes (1 axle only) were smoking. And I had to hydraulically disconnect an entire axle set of brakes to prevent complete brake destruction. Invest in an infrared non contact digital thermometer; you will not be disappointed in how many different diagnostic things it can help with.
Brake Pad to Caliper Corrosion. The pads must not be stuck on the calipers. Cleaning the caliper pad mount surface with a stiff wire brush and removing any rust scale, then a light film of brake caliper grease on the pad to caliper mounting ears.
Brake Fluid Flushing. Recommended to flush out fluid that might be moisture contaminated to prevent future corrosion. Piston to caliper corrosion can cause brake drag, overheating and failure. We never did hear a final from HAP3 on his catastrophic hose failure
We obviously need dependable powerful brakes. The trailer came with the EOH and I just wanted it to be as good as possible, hence many of the DOT 3 bleeding sessions. I really think I didn’t come up with a secret fix; I just didn’t give up until I was satisfied with the brakes. And we rely on hydraulic disc brakes for just about every modern passenger car and light truck braking system. They are proven performers with uncountable road miles of service.
Please use caution and common sense; these are your brakes we are discussing. I made my own decisions, and only offer my opinions and actions.
Both of these threads also have significant disc brake observations and owner input. I wish HAP3 would have advised on his outcome in the total brake failure.