Braking – Electric Vacuum Pump

Hella UP28 Electric Vacuum Pump

I have used brake components from various sources and I have no idea how good the brakes will work or feel once complete, therefore I’m using a Hella UP28 Electric Vacuum Pump.

We all like responsive brakes with a good feel, but modifying engines and brakes can render them less effective. Loss of manifold vacuum will have a negative effect on servo performance, leaving the brakes with a very hard feel. Although still effective, the brake pedal will need considerably more pressure to activate.

Manifold vacuum is created by the piston trying to suck air/fuel into the cylinder whilst the throttle is closed or partially open. With the throttle fully open there is very little vacuum. High lift cams, turbos, superchargers and free flow inlets can all reduce the amount of vacuum generated. The amount of servo assistance you get is dependent upon the vacuum available and chances are you’ll need maximum breaking effort whilst at full throttle.

Hella UP28 Electric Vacuum Pump

My daily driver is a Seat Cupra Turbo. Like many Audi, VW, Skoda, BMW etc it uses the UP28 electric vacuum pump to boost servo vacuum when needed. A standard production engine will produce between -18 & -22inHg (-8.8 to -22Psi) of vacuum at idle. My particular model is 1K0612181F.

The service manual specification for the brake booster vacuum is 500mmHg / 19.7inHg (10psi).


Up28 Electric Vacuum Pump

I’ve read on a couple forums that this pumps reliability is affected by vibration, so mounting it using the factory rubber mounts and a solid bracket are recommended.

How do I know whether I need a vacuum pump?

  • Disconnect the vacuum hose from your servo and connect it to a vacuum gauge
  • With the engine warm, check the vacuum at idle
  • Have a surf on you Servo’s part number to see if there are any published specifications and minimum required vacuums. If none is found use 20inHg (9.8Psi).
  • Check that you have enough manifold vacuum

Parts List

Because the UP28 is rated at 15amps max, I’ve included a 30A relay and a 20A fuse.

UP28 wiring diagram

Vacuum Pressure Switch

Multicomp PSF109S

psf109s pinout
The PSF1095 vacuum pressure switch is from -5.9 to -22.4inHg (-2.9psi to -11.9psi). Hopefully, that should allow me to get a good compromise between pedal feel and the desired servo vacuum. I’ve kept all the hoses short a taken the vacuum switch close to the servo.

You could also use the VAKUSW-18-B, adjustable from -5.9 to -28.9inHg

Checking the system

  1. Remove the relay
  2. Turn the vacuum pressure switch fully clockwise to -22.4inHg (-2.9Psi)
  3. Check the 12v feeds to the relay. You should have permanent power at pin 30 and 12v at 86 with the ignition on.
  4. Turn off the ignition and insert the relay
  5. Turn on the ignition
  6. Check the pump is running
  7. Check for vacuum leaks
  8. By turning the dial anti-clockwise you should be able to get the pump to turn off
  9. Turn the dial fully clockwise.
  10. Start the engine and let it idle
  11. Slowly turn the dial anti-clockwise until the pump turns off
  12. Depress the brake pedal – the pump should start again
  13. If the pump has a tendency to turn on and off, then turn the dial slightly anti-clockwise to increase the hysteresis
  14. With everything connected and running satisfactory test your car in a safe place

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