As with any closed hydraulic system, the safe functioning of a vehicle braking system depends on it being free from air.
That means that the entire braking system has to be bled after replacing any hydraulic component of the brake. In addition to the mechanic's expertise, this requires good workshop equipment with the corresponding bleeding devices.
TRW offers bleeding devices in two sizes and an adapter case for the different connections on the fluid container.
Despite all efforts, it happens regularly that the brake cannot be bled sufficiently or only bled properly after several attempts. Things can then become particularly difficult if new brake calipers with an integrated manual brake mechanism were installed. This applies to mechanical systems as well as to calipers with an electric park brake (EPB). The reason for this is the complex internal structure of these brake calipers.
The numerous components required for actuation of the parking brake independent of the hydraulic function create many chambers and cavities from which the trapped air is very difficult to remove (areas marked in green).
In this case the vehicle manufacturers stipulate different bleeding procedures. For example, the caliper has to be connected to the brake hose before installation and then rotated several times during bleeding. In addition to this, the system has to be depressurised between the individual work steps.
That costs time and money.
The TRW Service Information XZS151 summarises this procedure for a caliper with electric park brake.
Pre-filled TRW calipers for the spare parts market
This Service Information was created for a vehicle manufacturer supplied with unfilled brake calipers. For spare part sales, we supply pre-filled brake calipers as a rule. The video shows the filling system and the filling procedure. This involves injecting an amount of brake fluid precisely adapted to the caliper into the brake caliper under pressure. The caliper is then sealed with a grub screw. This ensures quick and easy bleeding after replacing the part – a clear advantage over unfilled brake calipers.
The TRW Service Information XZS117 reminds the mechanic to remove the sealing screw only just before connecting the caliper to the brake line or the brake hose.
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