Pre-holiday check: marginal tyres, a reset TPMS and a pull to the right

Almost every item on this list is a reason not to leave as the car stands:

1. Rear tread at 1.6 mm. That is exactly the legal minimum (at least 1.6 mm across the central three-quarters of the tread and around the whole circumference), so the tyres are legal by a hair and nothing more. Replace them.
2. Cracking on six-year-old rubber. Age hardens the compound and reduces grip in the wet regardless of tread depth. Tyre makers generally advise inspection from around five years and replacement by ten; with visible cracking, replace now.
3. TPMS reset without checking pressures. The system compares wheel speeds against stored values. Resetting told it that the current, possibly wrong, pressures are correct, so a slow puncture can now go unnoticed. Measure with a gauge, inflate to the loaded pressures, then reset, and re-check after the first 100 km.
4. Pull to the right. Could be uneven pressures, a dragging caliper, worn bushes or alignment. Any of these gets worse when hot, loaded and towing.
5. Brake fluid at four years. It is hygroscopic; absorbed water lowers the boiling point. With a trailer on a 30 °C day that is exactly the fade scenario you do not want.

The 1.6 mm figure is a floor set by law, not a target set by physics. Wet grip falls away sharply as tread depth drops below about 3 mm, because the grooves can no longer clear water fast enough: braking distances on a wet motorway grow by several metres and aquaplaning starts at lower speeds. You are adding three multipliers that all push in the wrong direction: 30 °C heat, a fully loaded car and a 1,200 kg trailer pressing on the rear axle. The rear tyres are also the ones that keep a towing combination from snaking.

On pressures:

• Use the handbook's full load plus trailer figures, which raise the rear notably above the normal-load value.
• Measure cold, before the car has been driven far, and never bleed a hot tyre down to the cold figure; the pressure rise with heat is normal.
• Do not exceed the maximum pressure moulded on the sidewall.
• Underinflation is the real enemy with a heavy load: the sidewall flexes more, heats up and can fail at motorway speed.

So the honest answer is that the rear tyres should be replaced before the trip, not merely pumped up.

A constant pull that survives a pressure correction points to something mechanical, and most of the likely causes deteriorate with use:

• A dragging brake caliper overheats, which is the first step towards fade and, eventually, towards the wheel locking or the car veering under braking. It also ruins fuel economy and cooks the fluid.
• Alignment or worn suspension bushes scrub a shoulder of the tyre rapidly. On tyres already at the legal minimum, that shoulder can reach the cords within a few hundred kilometres, and a blowout at motorway speed with a trailer attached is very hard to recover.
• A trailer amplifies every instability. A car that wanders slightly on its own will snake with 1,200 kg behind it, especially downhill or in crosswind.

So have it checked on a brake tester and an alignment rig before you load up. Also confirm the towball nose weight, the trailer's tyre pressures and its lights and breakaway cable, because the trailer is the part of the combination you have probably looked at least.