Simple upgrades to boost trailer braking

According to recent research, less than one-fifth of farm trailer brakes are road legal. Nick Fone and Oliver Mark take a look at a few ways to get your stopping power up to scratch.

Trailer braking – like school exams, is a subject that triggers bouts of extreme boredom and the prospect of abject failure, or that’s what some pretty startling survey results have revealed recently.

Out of 310 trailers tested over the past two years by dealer organisation BAGMA, only 12-15% could generate the legal minimum braking force. That would suggest that, like pre-exam revision, most of us can always find something more interesting/pressing to do than brake maintenance.

Like wafting honey under the nose of a hibernating grizzly, with such extreme levels of un-roadworthiness, we’re just asking for trouble from The Law.

While BAGMA’s headline figures might be pretty shocking, it’s even scarier when you drill down into the detail.While you might expect high-spec, high-speed trailers with air-brakes to sail through the tests, those were the ones that actually fared worse. Some 78% failed to achieve the minimum 45% braking performance for commercial-specification braking systems, while 10% had no brakes at all.

Lower-spec trailers with hydraulic brakes, however, only have to achieve a 25% braking efficiency. Nearly two-thirds still failed and a quarter of those tested lacked any braking ability whatsoever.

braking - setting-lever

With farming margins tight as ever and machinery prices in a seemingly unending upwards spiral, a new trailer is probably going to be an expensive bet.

In an effort to improve things, BAGMA has come up with a list of simple, (relatively) low-cost things you can do to upgrade your existing brakes.

Brake upgrade – Bagma suggestions
  • Larger (3/8in or 1/2in) diameter hydraulic brake hose £100-£150
  • Larger (25mm – 35mm) brake rams £120-£260
  • Replacement lever arms (including manual slack adjusters) £120-£150
  • Hydraulic breakaway failsafe system £300-£450
  • Parking brake improvements £100
  • Hydraulic load sensing £280-£450
  • Replacement commercial spec. axles £2,000-£2,500


The law – a few reminders
  • Maximum gross train weight 24.39t
  • Maximum speed 20mph (32kph) – only vehicles such as the JCB Fastrac and the Mercedes Benz Unimog with sprung axles front and rear may travel faster than this
  • Minimum braking efficiency – hydraulic brakes (up to 20mph): 25%
  • Minimum braking efficiency – air brakes (above 20mph ABS and dual-line failsafe system required – can also be hydraulic): 45%
  • Maximum gross weight of a conventional drawbar (unbalanced) trailer: 18.29t (even if the manufacturer’s plate says otherwise)
  • Maximum drawbar loading: 3t
Breaking contacts
Totrax – 01205 280 578


TractAir – 01482 576 222


Erentek – 01522 720 065


ADR Tyremart – 01400 283 820


BPW – 0116 281 6100


GES – 01733 210 021

Granning Axles

Hunton Legg – 01728 663 010


J.H. Milnes – 01226 766251


We wanted to find out just what effect BAGMA’s recommendations would have, so we put them to the test.

We chose a 10-year-old 12t AS Marston Ace as our test candidate, as it’s been a perennial best-seller for decades and there’s a big fleet of them out there. With 400mm diameter drums, 80mm wide shoes, 20mm rams and 1/4in pipework, it’s pretty standard ag spec.



Before touching anything we gave the trailer a run in its found-on-farm state. Unsurprisingly, it failed pretty convincingly, with just 14% braking efficiency.

The brakes on this standard spec 12t AS Marston Ace have a conventional lever arm with a split end to couple it to the drum’s splined cam shaft. This shaft twists to squeeze the shoes tight against the inside of the drum.

Thanks to regular maintenance the levers weren’t seized in place, so we were able to loosen them and knock them off, cranking the cam shaft right round with a set of Stilsons until the shoes made contact with the drum and then refitting the lever. With that done we re-attached the ram using the right hole in the lever to get the angle between the two as close to 90˚ as possible, to allow the ram to exert maximum force.

Re-test result: 16.2% braking efficiency (+15%)



Like many of its counterparts, new a decade ago, our test guinea pig had pretty lean 1/4in pipework. We decided to give it a serious upgrade, going straight for 1/2in hose and fittings.If you get technical about it and work out your Pi R-squared sums, that equates to an increase in bore of almost 300% – a serious increase in oil flow.

Re-test result: 15.4% braking efficiency (-4%)

Not a good result – bigger hoses with smaller rams do not necessarily equate to better braking.



Again pretty much standard fitment for a trailer of this vintage, 20mm rams look pretty puny when lined up alongside their 30mm equivalents.We whipped off the old ones and welded these on in their place, being careful to maintain a 90˚ lever-to-ram angle.

Re-test result (1/4in hose): 33.6% braking efficiency (+107%)

A huge leap in performance, even with the old pipework.

Next attempt with the bigger hose:

Re-test result (1/2in hose) – 38.3% braking efficiency (+136%)