by Jesse Crosse
Published at HAGERTY, Insurance for classic cars and motorbikes, with the friendly support of Calum Brown.
We’ll start one with a caveat. Any work on braking systems should be approached with maximum care, especially a classic which unless it’s the modern variety, won’t have ABS, safety systems to warn of an impending failure and may not have dual circuit braking. That said, brake callipers are simple enough to overhaul, the main difficulties being extracting pistons or bleed nipples which have seized through age and weathering.
Callipers come in lots of shapes and sizes and are designed to match the weight and performance of the car they’re fitted to. Essentially, they all work the same way. When the driver presses the brake pedal, brake fluid is forced into the calliper body, forcing pistons against the brake pads and the pads against the disc. Callipers straddle the disc and grip it pincer-like with the pads to slow the car.
The two main types you are likely to encounter are floating and fixed. Floating callipers only have one piston acting on one side of the disc. It ‘floats’ on pins so the casting can move relative to the disc. As the piston extends and puts pressure on one pad, the other side of the calliper casting is pulled against the opposite side of the disc, applying the other pad. With a floating calliper, the body of the calliper moves when braking.
Fixed callipers comprise a single casting which straddles the disc with opposing pistons each side of it. It may have one piston per side, two or even on three on very high-performance cars. On a classic, the most you’re likely to encounter is two per side (known as a four pot calliper) or more likely, one per side. The fixed calliper is bolted to the suspension knuckle or strut and only the pistons move.
Whatever the case, rebuilding is pretty similar. With the calliper, taking care to either clamp a flexible hose attached to it (as long as it’s not the steel braided variety) or using a bowl to catch the draining fluid, the first job is to clean it. The object of the exercise is to avoid any particles of dirt getting inside the caliper so treat it like a surgical procedure. Away from the clean area you’ll use for reassembly, clean the calliper externally before dismantling it. Start with degreaser, soap and water if it has baked on dirt, then brake cleaning fluid and a wire brush.
Remove the rubber dust seals which clip onto the casting and each piston, then you’re ready to remove the piston or pistons. The best (and sometimes only) way to do this off the car is with a compressor with the bleed nipple tightened and using a simple blowgun. Bear in mind that the piston can exit the cylinder like a bullet from a gun, especially if it’s seized and higher pressure is needed to shift it. It can cause damage both to the calliper and you, so it’s essential to take steps to trap it when it does decide to move.
A simple solution is to put a block of wood where the pads would sit in the calliper, leaving enough space for the piston to move but not enough that it can come right out of its cylinder. You can then blast away with impunity. Once it’s part way out, you can reduce pressure and use a thinner piece of wood for it to fire against. With two-or more pistons in a fixed piston calliper it’s slightly trickier because with one piston out, there will be no way of building pressure for the others. The trick is to inhibit all but one of the pistons using the same technique and free-off each one in turn until they can be removed.
Once you have all the pistons out, inspect the bores of the casting. Because the seals are usually fitted in a groove in the bore of the cylinder, rather than on the piston, these will usually be recoverable after a good clean. Seized pistons will nearly always need replacing and rebuild kits are usually easy to find. Never reassemble a calliper with anything but perfect pistons and new seals.
Give the body a final clean until it’s immaculate, then assemble the hydraulic seals and pistons with a smear of brake fluid. Never use oil or grease inside a braking system, anywhere. Once the pistons are in place, coat the inside of the dust seals with the red rubber grease usually supplied, and fit those. Most have a steel circlip to hold them in place. You can replace other parts too, like pad retaining pins and anti-squeal plates for a tidy job.
If a car has been standing for ages and the bleed nipple is seized, soak it overnight with penetrating fluid while the calliper is stripped and warm it up with a blow torch if necessary. Then use a socket to avoid rounding it off, but be super careful – shearing one off will seriously spoil your day. Renew the bleed nipples as part of the rebuild as well and always rebuild both callipers on the same axle at once for obvious reasons. Once the job is done, stopping power and stability under braking should be improved or even transformed making those Sunday jaunts safer and a lot more fun.
Written for by our friends ,Hagerty Insurance, by Jesse Crosse who started as a motoring hack in 1982, was launch editor of Performance Car magazine and signed up an unheard writer called Jeremy Clarkson. He now writes about automotive technology, and spends his time restoring a pair of fast Fords, a 1968 GT390 Mustang fastback, and the same Ford Sierra Cosworth long-term test car he ran while editor of Performance Car.