Thread: Towing braked trailers

Originally Posted by No.3

Part of the reason for this is that the 2500Kg master cylinder is only rated to operate one braked axle or one set of disc's. A 3500Kg trailer is required to have both axles braked and the override coupling can't push that much fluid to operate all four calipers without making the WoF man think that the brakes are spongy and need bleeding... Ask me how I know this haha (oldmate thought it would be a 2 is good, 4 must be better in terms of numbers of calipers on his braked tandem boat trailer). It did stop well, I'll give it that but we could not get it past WoF once the little Indian inspector killed it for spongy brakes. The brakes passed in every other respect, good stopping power etc etc it just was too much fluid required to push out all four pistons compared to the available volume in the master cylinder. We actually looked at custom fitting a double master cylinder setup onto the coupling, but that wasn't allowed and also custom building a larger master cylinder but again not a goer. Ended up either needing to drop back to two calipers or only one braked axle or what we did in the end and fitting a US made electric/hydraulic actuator with wireless controller.

Standard override has a 3/4" master cylinder, you need to fit a 7/8" mc to drive 4 calipers effectively. Though its bloody hard to bleed every last bit air to get rid of a bit of sponge. Still only rated to 2500kg on override, with 4 wheel braking.

Originally Posted by Mathias

Standard override has a 3/4" master cylinder, you need to fit a 7/8" mc to drive 4 calipers effectively. Though its bloody hard to bleed every last bit air to get rid of a bit of sponge. Still only rated to 2500kg on override, with 4 wheel braking.

This. Ive done plenty of trailers with dual braked axles. You must have the bigger master cylinder.

Originally Posted by Mathias

Standard override has a 3/4" master cylinder, you need to fit a 7/8" mc to drive 4 calipers effectively. Though its bloody hard to bleed every last bit air to get rid of a bit of sponge. Still only rated to 2500kg on override, with 4 wheel braking.

Yes, but the fly in the ointment is that overriding all the above is that you have to be able to stop in 7m at 30km/hr.

Originally Posted by Mathias

Standard override has a 3/4" master cylinder, you need to fit a 7/8" mc to drive 4 calipers effectively. Though its bloody hard to bleed every last bit air to get rid of a bit of sponge. Still only rated to 2500kg on override, with 4 wheel braking.

No, was running 7/8 and still not good enough. I know what the recommendations are - don't know why it didn't work in this instance. Properly vacuum bled and fully backfilled system with the front of the trailer lifted into the sky and any air traps were straightened to release all air from the system - but it just couldn't move enough fluid without the piston in the master cylinder having to move to the amount that gave the inspectors the spongy feel. There was nothing 'wrong' with the system, no leaks, no air, no mechanical problem and it passed the test just fine in terms of braking effectiveness - it just literally didn't move enough fluid ina short enough distance to make the inspector happy with the operation of the master cylinder. Needed to go to a full 1" we reckoned to get it to the point of success which is a foreign product - but in the end of it to strengthen the trailer to the point of not killing duratorque axles every other year and fix a few other issues it basically went past the 2500Kg rating limit of the override coupling, so with all of those other issues in the mix it was a case of "lets use the ton" and fix everything including a electric/hydraulic actuator. Went from spongy to locked up in f-all and a WoF inspector that tried to fail it because he'd never seen one before and couldn't drive it (facepalm). Was good after that though.