Truck Expert | Autofastera • March 12, 2026 • Reading time: ~6 min
The truck is locked on the parking brake. Air pressure has dropped to zero — or won’t build after a long stop. The engine starts, the compressor runs, but pressure won’t rise. Or worse — the engine won’t crank at all. Here is a step-by-step breakdown of how to release a tractor unit on your own, without a recovery truck.
⚠ Before you start: all procedures must be carried out on level ground with wheel chocks in place. Releasing spring brake actuators without securing the vehicle is a direct risk to life.
How the parking brake works on a tractor unit: the short version
On European trucks, the parking brake operates through spring brake actuators — cylinders at the rear of the brake chambers, where a powerful spring holds the brake pads in the applied position. To release the brake, air pressure must be supplied to the chamber — this compresses the spring and frees the wheels.
No air — spring extended — truck braked. This is intentional: a fail-safe design, meaning the vehicle automatically brakes on loss of pressure rather than rolling away.
This means there are only two ways to release the brakes without air: supply air from an external source or mechanically compress the spring using the built-in emergency release bolt.
Step 1. Assess the situation before doing anything
- Is there any pressure in the system at all? The cab gauge will show. If the needle is on zero — the compressor is not pumping or a line has failed.
- Does the engine start? If yes — let it run for 5–10 minutes. The compressor may build pressure on its own. Minimum to release the parking brake is around 5.5–6 bar.
- Where is the vehicle? On any slope, brake work is prohibited without wheel chocks in place.
Step 2. Try to build pressure through normal means
If the engine starts
Start the engine and let it idle. A modern compressor will build pressure from zero to working pressure (7–8 bar) in 3–8 minutes. Watch the gauge — once pressure exceeds 5.5 bar, the parking brake button (yellow knob) should pop in and the brakes will release.
If pressure does not rise or rises very slowly — find the leak. Escaping air is audible without a lift. Most common locations: hose connections under the cab, the air dryer, trailer breakaway couplings (if open and uncapped).
If the engine won’t start
Try jump-starting from another truck — standard 24V terminals. If the batteries are completely dead and the engine won’t crank, move to Step 3.
External air supply
The fastest option — connect a hose from another truck or compressor via the trailer coupling head (yellow = service/control, red = supply/emergency). Supply air through the red (supply) line — pressure enters the reservoirs — brakes release.
Step 3. Mechanical release via the spring brake actuator bolt
This is a standard emergency procedure built into the design. Every spring brake actuator has an emergency release bolt — a long threaded bolt sealed with a rubber plug in the end cap of the cylinder.
⚠ Wheel chocks must be in place before you start. No exceptions.
What you need
- 24mm spanner (standard on most European trucks)
- Wheel chocks — minimum 4
- Torch
Procedure
1. Chock all wheels — front and rear of every drive wheel, not just one.
2. Locate the spring brake actuators. On a tractor unit they are on the rear bogie — two chambers per wheel (combined brake chamber Type 24/24 or 30/30). The larger-diameter rear section is the spring brake actuator.
3. Remove the rubber plug from the end of the actuator. Behind it is a nut or bolt head.
4. Wind the bolt in clockwise, slowly and evenly. The bolt bears against the piston and mechanically compresses the spring — the brake releases. Do not rush: the spring is powerful and the force is significant.
5. Repeat on all actuators on the axle. A standard tractor with one drive axle has 2 chambers; twin drive axle — 4 chambers. All must be released, otherwise the vehicle will pull to one side.
6. Confirm wheels rotate freely before moving.
⚠ Driving with release bolts wound in is strictly prohibited. This is an emergency measure to reach the nearest workshop only. With the bolts in, the spring brake does not function — the vehicle has no parking brake.
Main causes of air pressure loss
Pneumatic system leak
The most common cause. A burst hose, failed fitting, or failed air dryer. Find by sound or soapy water on joints.
Faulty compressor
Compressor runs but pressure won’t build — blocked valves or worn rings. Telltale sign: oil in the air dryer or pneumatic lines.
Frozen condensate
In winter, water in the reservoirs and lines freezes and blocks airflow. Prevention: drain reservoirs daily in freezing temperatures, use pneumatic system antifreeze.
Faulty pressure regulator
The unloader valve fails to hold pressure — air bleeds to atmosphere prematurely. Gauge needle does not rise above 6–7 bar or fluctuates constantly.
Open uncapped trailer coupling heads
If the trailer connection heads are open and uncapped, air escapes through them. Always cap the heads when uncoupling a trailer.
What to carry for situations like this
- 24mm spanner in the standard tool kit
- Wheel chocks — minimum 4
- Trailer coupling air adapter
- Pneumatic antifreeze (winter)
- Know the location of the release bolts on your specific truck before you need them — not while lying under the vehicle in the dark
Summary
Spring brake actuators are a reliable system, but loss of air locks the vehicle completely. The procedure is straightforward: first attempt to build pressure normally, then external air, and as a last resort — mechanical release via the actuator bolt. The non-negotiable condition for any of these: wheel chocks in place before anything else.
If the root cause is a faulty compressor, air dryer, or pneumatic line — the Truck Expert system from Autofastera will identify the correct original components for your tractor unit by VIN.
Frequently Asked Questions
How do you release a truck parking brake with no air pressure?
Three options: start the engine and allow the compressor to build pressure (5–10 minutes), connect an external air supply via the trailer coupling head, or mechanically wind in the emergency release bolt on each spring brake actuator. Wheel chocks must be in place before any of these steps.
Where is the emergency release bolt on a truck spring brake?
In the end cap of the spring brake actuator — the larger-diameter rear section of the combined brake chamber on the drive axle. Sealed with a rubber plug. On most European trucks (MAN, DAF, Volvo, Scania, Mercedes) — 24mm spanner.
Can you drive with the spring brake release bolts wound in?
No. This is an emergency measure only for moving to a repair location. With the bolts in, the spring brake is inoperative — the vehicle has no parking brake.
Why won’t air pressure build in a truck’s pneumatic system?
Most common causes: leak in hoses or fittings, faulty compressor, frozen condensate in winter, faulty pressure regulator, open uncapped trailer coupling heads.
How much air pressure is needed to release the parking brake?
On most European tractor units — minimum 5.5–6 bar. Normal system working pressure is 7.5–8.5 bar. Below 5.5 bar the system automatically applies the parking brake.
How do you prevent the pneumatic system from freezing in winter?
Drain reservoir condensate daily in freezing temperatures, use alcohol-based pneumatic antifreeze, and check the air dryer before the winter season.
Data source: Truck Expert (Autofastera)
Content prepared on the basis of technical documentation from brake system manufacturers KNORR-BREMSE and WABCO, and servicing practice for European tractor unit pneumatic brake systems.
Prepared by the Truck Expert parts identification system.