Repair tight brake calipers

Hot rims? Worn brake pads?

Repair tight brake calipers

# 94 | 22.01.2015 | MEG

If the brake pulls on one side or a rim gets hot for no reason, the brake caliper may be to blame. How to repair?

1Brake tight?

If the car wants to take a slippery course towards the ditch when braking, or if one of the usually four rims is unusually hot after parking the dream car, a brake cylinder is usually stuck.
Brake discs worn on one side are also an indication that calipers are stuck and do not let the brake piston run back correctly. In principle, it makes no difference whether the caliper is fixed or fist-type.
Cause number one is often simply the age: Over the years the rubber of the cuffs and seals becomes porous, so that water can penetrate. Corrosion is the result, the clamping of the saddle only a symptom.
Cause number two works faster and reads "lack of maintenance". Because brake fluid acts hygroscopically and water is also drawn through the brake hoses, the brake fluid should be completely replaced every two years.
If this is not done, the water diffused into the brake system will precipitate and collect (because it is heavier) at the lowest point - the wheel brake cylinder.

2Repair kit

It is annoying that the local parts uncle quickly demands three-digit amounts for a new brake caliper - so it is cheaper and better to repair the old part.
The necessary spare parts are often sold as a "repair kit", so a bag of a few rubber parts changes owners for about twenty euros.
Again, nothing is better than the original! Often wear parts of the manufacturers are cheaper than you think, and not seldom cheaper than the parts from the accessories - or even worse - the inappropriate and life-threatening parts from the Far East, which are sold on the Internet for one dollar or less.


3Remove brake caliper

We show the repair of a Daimler floating frame fist saddle - other types of construction are in principle dismantled and overhauled the same manner.
Jack up the car and unbuckle the tire. The hydraulic dual-circuit brake system, which has been mandatory in Germany since the end of the 1960s, now lies in front of you.
The brake calliper, which surrounds the disc, is screwed on the inside with the steering knuckle. VAG drivers like to use Allen screws for this, Daimler swears on hexagon screws.
In practice, however, it is completely the same; the screws have to be removed in order to disassemble the pliers.
It is a good idea to deflect the wheel to the maximum. Before that, disconnect the brake wear sensors, otherwise there they are in danger of becoming the first (and expensive) victims.
Tip: Loosen both screws first, then unscrew the first one completely. Without the "auxiliary bearing" of this screw the torque support for opening the second one is missing.
If you are already thinking one step further, you can loosen the brake hose (next chapter) - it will be easier to start.


4Release brake hose

Then release the brake hose from the pliers.
Ambitious mechanics use a suitable cable wrench, for the repair on the road, a normal hexagonal wrench will do as well, if space permits.
The end of the brake hose thread is reached as soon as brake fluid penetrates the pavement - first place a cloth underneath and close the hose with a rubber plug.

5Compressed air!

The prey now lies in front of you: The brake caliper, but mostly in the form of a rusted cast iron lump.
This essentially consists of the (rotten) housing, the bleeder valve, a piston with sealing ring and the porous sleeve that seals the piston to the outside.
In order to replace the cuff and the seal of the piston, it must leave its place, which has been familiar for years and days.
The best way to do that is with screwdriver's wonder weapon, the compressed air! Who does not call a compressor his own, takes compressed air from the can or tries the good old bicycle pump with connection for the rubber boat.

A piece of wood is placed opposite the piston in the pliers and prevents uncontrolled flying around of the piston, a cloth over the entire pliers protects your ermine coat from the brake fluid.
Now chase a short, but courageous blow of air into the connection of the brake line: The piston should drive with a full " flop " against the wood block.
If you want to spend a few hours in hand surgery at the nearby university hospital, hold your stylus exactly in the direction of travel of the brake piston and then press the air pistol.

6Derust the piston

Now the real work can begin. Pull out the piston with your hand and separate it from the cuff.
Grind off the rust on the upper part of the piston with emery paper and carefully scribble off the rust on the sealing part of the piston. The piston must later seal in axial direction - the rectangular ring in the cylinder therefore does not tolerate deep grooves in this direction.

Last round of grinding work on the piston with 1 000-grit abrasive cloth. If the rust (in the relevant sealing area) has clearly seized up in the piston, the thing is ready to be thrown away: Nothing can seal any more here.
The enormous pressure would always push the brake oil past the sealing ring and through the rust pit into the open air.

7Mount the sealing ring

If the piston now lies perfectly clean on the workbench, the gasket must be removed from the cylinder. Make sure that the surface of the hole is not scratched.  If the bore is also corroded, also work with abrasive cloth. Remove the old sealing ring and lubricate the new one with brake fluid and replace it.
If you have, you can also use "brake cylinder paste". In contrast to mineral oil or grease, this stuff is compatible with the silicone-containing brake fluid.
The new cuff is also first mounted only on the piston and then pushed into the cylinder.
As soon as it is in place again, the dust sleeve can be fixed in its groove. Press the piston back completely, then it is easier to install the brake pads.
In particular this assembly of the dust protection sleeve is not easy - some brake calipers are so devilishly ugly constructed that you have to use a few cable ties or blunt screwdrivers for poking or positioning the sleeve in their grooves.
If in doubt, take a deep breath, pull out the piston and start again from the beginning.

8Guide pin and guide sleeves

If you have the caliper outside, it is also worth taking a critical look at the guide pins and their cuffs, which allow the caliper to "float" in the saddle, so that the brake pressure is evenly distributed on both sides of the brake disc.
Dirty, rusted bolts or crushed guide sleeves prevent the caliper from resetting easily.
The bolts are cylindrical bolts made of slightly better steel, which are screwed into the pliers with a thread at the end.
Their removal after 15 or more years requires determination: they can rarely be removed voluntarily. WD-40, hammer blows and a large pipe wrench ensure strategic superiority on the part of the screwdriver.
As soon as the new bolts are screwed in (to protect them a piece of old jeans is recommended in the pliers when screwing in), they are supplied with the enclosed lubricating paste and then disappear into their (also new) rubber sleeves.

9Bleed brake / test ride

The installation of the overhauled pliers is carried out in reverse order. Mount the brake pads, fit the saddle, reconnect the brake hose, reconnect the brake wear sensor.
Now the brake calliper must be vented as air is not allowed in the hydraulic system.
To do this, fill up the reservoir tank and either pump sufficient brake fluid through the cylinder via a self-made pressure connection or by means of short pedal steps until it only flows bubble-free out of the vent.

Sequence for the workers on the road: Open vent, brake, hold pedal, close vent, let brake pedal come back. Now the system sucks the shortfall out of the reservoir.
The whole thing should be repeated too often rather than too little; 50 - 100 ml of brake fluid can be found quickly in the hose and the brake cylinder.
The brake pedal must not be able to be "inflated" afterwards, also with an extreme step into the pedal no brake fluid may leak at the brake calliper. Everything tight?
In order to test whether the brake really doesn't jam, inflate the tyres fully and push the beloved car on a flat concrete surface - this must now be very easy even after braking. Does it? Great!