Relay Valve vs Quick Release Valve: Which One Is Failing?
A relay valve applies the rear brakes from a tank when the pedal signals; a quick release valve vents chamber air at the axle when the pedal lets go. Soap both exhaust ports. On the service brakes, a leak with the pedal up comes from the relay valve, the only one holding tank air between stops.
Last updated: November 12, 2026
Key takeaways
- A service quick release valve holds no air with the pedal up, so an exhaust leak then is the relay valve's.
- Bendix rejects a QRN quick release valve leaking more than a 1 inch (25 mm) bubble in 3 seconds at 100 psi.
- A blocked exhaust on either valve traps air and drags the brakes.
- FMVSS 121 timing limits certify new vehicles; they are not a workshop pass mark.
A relay valve is a remote-controlled air valve: a small signal from the foot valve makes it send matching reservoir pressure to its chambers and later vent them through its own exhaust. A quick release valve is a simpler diaphragm valve with no reservoir feed that vents chamber air at the axle on release.
New York's CDL manual, Section 5, puts air brake lag at half a second or more, about 32 feet (10 m) of extra stopping distance at 55 mph (89 km/h) on dry pavement. Valves near the chambers shorten the lines that fill and empty; our guide to every valve in a truck air brake system shows the full layout.
Where does each valve sit, and what does it do?
Bendix's Air Brake Handbook, an older edition kept in an NHTSA investigation file, calls relay valves graduating valves of high capacity and fast response, used mainly on long wheelbase vehicles to apply and release the rear axle service or parking brakes. They mount close to their chambers, on the frame or directly on a reservoir. On trailers, an antilock modulator often takes the plain brake relay valve's place.
Bendix's QRN service data, SD-03-905, says a quick release valve speeds the exhaust from the actuators it serves and usually sits on the axle between its two chambers. The standard QRN passes an application with at most 1 psi differential: 10 psi in, 9 psi out. On spring brakes, the handbook's QR-1C vents the spring side and adds a double check valve, so service and spring forces never act on one brake together.
Relay valve vs quick release valve: how the two compare
| Point | Relay valve | Quick release valve |
|---|---|---|
| Job | Applies and releases chambers with reservoir air on a signal | Vents the chambers fast on release |
| Air at its inlet | Reservoir pressure at all times | Only during an application (service type) |
| Usual place | Near the rear axle chambers, frame or reservoir mounted | Between two chambers on an axle, or on a spring brake line |
| Exhaust leak, pedal up | Inlet valve or its seals | Not possible on a service valve |
| Exhaust leak, brakes held on | Valve seats or internal seals | Diaphragm or exhaust seat |
| Fault that drags brakes | Plugged exhaust, sticking piston | Plugged or frozen exhaust |
| Reference figures | ZF-WABCO 973 006 003 0: 8.5 bar maximum, 0.9 bar response | Bendix QRN: 1 psi differential; ZF-WABCO 973 500 000 0: 12 bar |
Those figures apply only to the part numbers named; check any other valve against its own maker's data.
What are the symptoms of a bad air brake relay valve or quick release valve?
Rear brakes that come on late
A slow relay valve lets the steer axle bite before the drive axles. Check the signal first: a kinked control line, a leaking fitting or weak rear-circuit delivery from the foot valve feels the same, and the Bendix handbook's checklist for sluggish applications starts with kinked lines and badly fitted hose ends.
Brakes that release slowly or drag
Both valves release through their own exhaust port. In Bendix's QRN, the diaphragm's outer edge stays sealed against the upper body on release, so chamber air has one way out; dirt or ice there leaves that axle's brakes partly applied and its drums or rotors hot. A relay valve with a sticking piston or a crushed silencer does the same.
Air hissing from an exhaust port
With the pedal up, a service quick release valve holds no air, while the relay valve keeps reservoir pressure at its supply port, so a leak then means a passing inlet valve or seal. Under a held application either valve can leak, and a spring brake quick release valve stays charged whenever the parking brakes are off. Rule out the chambers too: Appendix A to 49 CFR Part 396 lists an audible chamber leak, such as a ruptured diaphragm, as a defect.
An ABS relay valve that stays silent
An ABS relay valve is a relay valve with antilock solenoids built in, or a modulator fitted in its place; in normal stops it brakes like a plain relay valve. Warning signs are no short puff of air at power-up and an ABS lamp that stays lit; Appendix A lists a lamp that never lights at power-up, or never goes out, as a defect.
How do you test both valves on the truck?
Chock the wheels, build to governor cut-out, stop the engine and release the parking brakes. The steps follow Bendix's QRN data, its handbook leakage test and Ford's Safety Recall 12S28 procedure for the Bendix ATR-6 traction relay valve on 2011 F-650 and F-750 trucks.
- Pedal up. Soap the relay valve exhaust and fittings. Ford accepts a bubble up to 1 inch (25 mm) in 3 seconds; a faster leak points to the inlet valve or an O-ring.
- Brakes held on. Hold an 80-90 psi application, wedging the pedal as the handbook suggests, and soap both exhausts and the relay valve cover joint. Bendix allows no leak between the QRN body halves and at most a 1 inch bubble in 3 seconds at its exhaust at 100 psi (689 kPa); Ford uses the same exhaust limit for the relay valve.
- Leak-down. Let the gauges settle, then watch them for 2 minutes. The handbook allows 4 psi on a single vehicle, 6 psi with one trailer and 8 psi with two; fix bigger losses first.
- Release. Let the pedal up: each valve should give a short, sharp exhaust, and Bendix expects QRN-fed chambers to return promptly to zero.
- Gauge check. Tee gauges into the relay valve's control line and a delivery port. Above the response pressure, delivery should track control; if it lags or stops short with control and supply proven, the valve is at fault.
- Power-up check. On an ABS relay valve, switch the ignition on with the system charged; a helper should hear or feel a short chuff, and the ABS lamp should light briefly and go out.
Judge another maker's valve by the leakage limit in its own service data.
What do the FMVSS 121 timing limits mean for these valves?
FMVSS 121 times a new vehicle from the first movement of the brake control. From 100 psi in the service reservoirs, every chamber on a truck or bus must reach 60 psi within 0.45 seconds, and from 95 psi it must drop to 5 psi within 0.55 seconds; trailers get 0.50 to 0.60 seconds to apply and 1.00 to 1.20 seconds to release. Relay and quick release valves help long air lines meet those figures.
These are certification limits measured with instruments, not a pass mark for a dash gauge; in the shop, compare chambers and axles with each other. An antilock signal fault must not lengthen those times on tractors built from March 1, 1997 or single-unit vehicles from March 1, 1998.
Should you rebuild or replace the faulty valve?
Many designs can be rebuilt. The Bendix handbook describes relay valves whose inlet and exhaust cartridge comes out without removing the lines; Bendix fixes a failed QRN with a genuine kit or a new or remanufactured unit, though the QRN-2 cannot be serviced. ZF-WABCO's product catalogue lists diaphragm kit 973 500 003 2 for its 973 500 000 0 quick release valve. Bendix's rule: parts with stripped threads or damage are replaced, not repaired.
For a replacement, match function, threads and pressure rating. The VADEN 303.01.0002 relay valve replaces ZF-WABCO 973 006 003 0, DAF 1518267, MAN 81.52116-6074 and Mercedes-Benz 004 429 63 44, with M22x1.5 VOSS ports 1 and 2, an M16x1.5 VOSS port 4, a silencer and an 8.5 bar maximum. The VADEN 301.07.0001 quick release valve replaces ZF-WABCO 973 500 000 0, has M22x1.5 ports and a 12 bar rating, and fits Volvo F10, F12 and FL10 and Ford Trucks Cargo models. Our relay valve catalogue and quick release valve range link to product pages with full OE cross-references; fitting steps are in our relay valve and ABS relay valve service guide.
Frequently Asked Questions
Can a bad quick release valve make the front brakes drag?
Yes. If dirt or ice blocks its exhaust, or the diaphragm will not lift off the exhaust seat, chamber air cannot escape and both brakes on that axle stay partly applied; the drums or rotors run warm after a short drive. Clear the port and repeat the held-application soap test.
Why does an ABS relay valve puff air when the ignition is switched on?
The short chuff is part of the antilock system's power-up self-check and is normal. Ford's ATR-6 recall procedure uses it as a test: with the system charged, a helper listens or feels for it as the ignition comes on. No chuff points first to the solenoid connector, then to the valve.
Does a relay valve raise or lower brake pressure?
A standard relay valve copies the control signal with more flow, without amplifying it. It needs a small control pressure before delivering: ZF-WABCO lists 0.2-0.3 bar for its 973 001 relay valves and 0.9 bar for the 973 006 003 0. Bendix's R-12P trailer and dolly relay has zero crack and differential.
Can a quick release valve be fitted where a relay valve was?
No. A quick release valve has no reservoir connection, so it cannot apply the chambers faster; it only releases them faster. A relay valve needs a reservoir feed, a control line and enough flow for its chambers. Replace like with like, comparing port threads, maximum pressure and any balance or anti-compound port.
How often should relay and quick release valves be checked?
Bendix sets a minimum for the QRN: an inspection for proper operation every 12 months or 3,600 hours of operation, whichever comes first. Its handbook recommends running five air system tests every quarter and filing the results, so a slow leak or lagging axle shows up as a trend.
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