Spring Brake Chambers Explained: Tristop Types, Sizes and How to Order

Spring brake chambers sit on almost every air-braked truck and trailer axle, yet the markings on them confuse a lot of buyers. WABCO's combined version is branded Tristop, and understanding what the type numbers mean and how to handle these units safely makes ordering the right replacement straightforward. Here is what you need to know.

Service chamber vs spring brake chamber

There are two related products, and it is important not to confuse them:

• A service brake chamber has a single diaphragm. Air pressure pushes the push rod out to apply the brake; releasing the air releases the brake. This is used on axles without a parking function.
• A spring brake chamber (the Tristop type) combines the service chamber with a second, spring loaded chamber. The powerful internal spring applies the parking and emergency brake; air pressure holds the spring compressed to release it. If air is lost, the spring applies the brake automatically which is exactly what makes it a fail safe parking brake.

What the Type number means

The number stamped on a brake chamber Type 16, 20, 24 and so on is the effective diaphragm area in square inches, which determines the force the chamber produces. A combined spring brake chamber carries two numbers written as service/spring, for example 16/24 (a 16 service side and a 24 spring side). Match both numbers when replacing a combined unit.

Standard vs long stroke

Chambers come in standard stroke and long stroke versions. Long stroke chambers give more available push rod travel, which keeps the brake effective as the linings wear. Standard and long stroke are not interchangeable, and long stroke units are marked to identify them (often by a stamped symbol as well as the type). Always replace like for like.

Reading the markings before you order

Before ordering, note down:

• The type (e.g. 24, or 16/24 for a combined unit) and whether it is long stroke.
• The part number stamped or labelled on the body.
• The mounting stud size and spacing.
• The push rod and clevis length and thread.
• The air port positions and thread.

Safety: never open a spring brake chamber

The parking spring inside a spring brake chamber stores enough energy to cause serious injury if the chamber is cut open or dismantled. Never disassemble one. To move a vehicle with no air (for towing or workshop access), use the release bolt (caging bolt) to mechanically wind the spring back and release the brake following the correct procedure and replace the whole chamber rather than attempting an internal repair.

How to order

Browse our WABCO range and brake elements for Tristop and service chambers. Send us the type, part number and stroke from your old chamber and we will confirm the exact replacement before shipping.

Need a quote? Send your parts list to sales@mspexport.com or message us on WhatsApp for a fast quotation.