Thursday January 30 2014

What is wrong with my Pajero’s brakes?

By Paul Kaganzi

My Mitsubishi Pajero has a faulty brake system problem. I have to kick the pedal as close as possible to the floorboard. A quick kick of the pedal gives firm resistance at first, but turns soft quickly and the pedal goes down to the floorboard. My usual mechanic is perplexed because the brake pads are okay and there are no signs of external leaks or abnormal fluid loss. The brake fluid reservoir shows no loss of fluid. Recently, I started hearing a whining sound like air escaping from the pinched neck of a balloon when the brake pedal is released after I stop. I would like to get your opinion on what the problem could be.
Kaggwa L.

The increased pedal stroke or reduced pedal to floorboard clearance you describe suggests that your Mitsubishi Pajero brake master cylinder may be faulty.
A brake master cylinder is a control device which converts non-hydraulic pressure into hydraulic pressure. It works by pressing on the brake pedal, which in turn pushes on the primary piston through a linkage.

Pressure then builds in the cylinder and lines as you depress the brake pedal further. The pressure that is between the primary and secondary piston eventually forces the secondary piston to
compress the fluid that is in its circuit.

If the brakes are operating properly, the pressure will be equal in both circuits. Bypassing is a leak that occurs internally within the master cylinder.

This is when brake fluid is being pumped into the front chamber out from the rear chamber and the rear reservoir would appear low on fluid. Bypassing can cause a low or sinking brake pedal feel. A defective sealing cup within the master cylinder would be responsible for this condition.

Since the master cylinder is mounted onto the brake booster, seepage may likely occur around the mounting area. A little seepage is normal.

But should a lot of seepage occur, the master cylinder will not be able to build sufficient brake pressure. I suggest your mechanic dismantles and inspects the master cylinder for signs of damaged or worn out seals, this will reinforce your decision to replace it.

My 1996 Toyota Land Cruiser VX has a persistent front differential box failure. I have replaced the differential twice with good used ones in a space of six months. However the last differential box we installed is also making that whining noise when I drive. My mechanic admits that the noise suggests this differential is damaged however he is reluctant to replace it again. Is it a case of buying poor quality differential boxes or could it be an installation problem?
Vincent Opolot

The short intervals between the replacement of two differential units suggest that something has been overlooked. A common cause of differential failure is leaking gear oil. Often technicians look out for leaking differential seals and ignore the hub seals. A differential is a device that splits the engine torque two ways, allowing each output to spin at a different speed.

The differential gears transfer drive to the wheels through the output shaft. The differential housing encases the planetary gear set and output shafts that attach to the wheel hub assembly. The differential planetary gears and output shafts rely on gear oil to lubricate and cool them during operation. On your Land cruiser there is a set of swivel hub or knuckle bearings that tend to wear and hub seals that begin to leak especially after 100,000kms, depending on the terrain usually travelled on. Damaged hub seals will lead to infiltration of grease into the differential housing which contaminates the differential gear oil which in turn damages the differential planetary gears.

The defective seals will leak differential gear oil and damage the differential gears as they run dry. It is possible that on the two occasions your mechanic has replaced the damaged differential boxes without checking the condition of the hub seals. Do ask them to dismantle the hub assembly and confirm the condition of the seals and swivel hub or knuckle bearings. My strong hunch is they are worn out and due for replacement. Consider getting another good used differential box after fixing the leaking seals. This time it should not fail.