Heavy vehicles like the MItsubishi Delica take some stopping so they come with hefty twin pot calipers. Brake pads flex hugely under braking pressure and with a single piston the pads tend to lift at the ends reducing the area in contact with the pad so reducing braking performance. The idea behind twin pot calipers is to control this lift by having two pistons, one at each end of the pad. Brembo Tokico and APR – performance brake calipers – have four or even six pot calipers to control the flex of the much larger brake pads used
On the Mitsubishi the caliper pins are prone to seizure for several reasons. The rubber boot around the caliper pin is not 100% water tight so for drivers who like to go off road water will eventually get onto the caliper pin and lead to corrosion. This will happen regardless of what lubricant is being used on the caliper pin if sufficient water is trapped for any period of time.
Use of a petroleum based grease on the caliper pin – copper,lithium – will swell the rubber boot and reduce the effectiveness of the boot to keep out water even further.
Corrosion on the pin will also occur in a matter of days after exposure to heavilty salted roads – the salt will also react with the lubricant and compromise it’s effectiveness. The only solution is to regularly take out the pin, clean and re-lube it. Happily in the case of the Delica and other large 4 x 4’s this is easy as there is enough clearance with the wheel on and steering on full lock the caliper pin bolts can be easily reached and take out. The rear bolts are also easily removed without taking off the tyres. Don’t over tighten them. (Take a look at the post Caliper Bolts – To Grease or Not to Grease.) The above picture shows a pin which has significant fretting corrosion exacerbated by ingress of water – see how the water has sneaked under the rubber boot. A good idea is to use some ProSlip PIN here just to help keep moisture at bay.
Lower caliper pins often have a small rubber bushing on the end. Too much lubricant (ProSlip PIN!) at the end of this pin will make it impossible to re-insert it and forcing it risks displacing the rubber bushing. Again using petroleum based grease will swell and render the bushing useless. Notice that the pin with the bushing has less fretting corrosion, this is because the bushing has acted to keep a distance between the metal surfaces of the pin and pin seat.