Lets be generous and say the brakes on the XK8 dont have a modern "feel".
So investigating I first measured the vacuum produced by the engine and it tured out to be only 14in HG . 22+ would have been satisfactory.
Stage One.
Fit an electric brake booster which will raise the vacuum to about 28 in HG in effect doubling the output from the brake servo. These are about £40 in UK ,fitted to Volvo etc and available in US.


Tight fit but it goes in .

I went for a simple system but failsafe. Foot on brake pump starts. Feed wire runs through to the brake light relay in the trunk fuse box. To get wire through drill a hole 1 in above the throttle cable inlet to the booster box ( to the outside of the booster) and use a grommet !
On the plumbing side run pipe (10mm) from pump through a grommeted hole to a T piece under the main engine air intake pipe and go to the brake servo one way and to the throttle body the other via a one way valve.
So if the pump fails the engine vacuum feeds the brake servo.


The arrow goes to the throttle body as we want to suck air out the brake servo.

Stage Two
While the pedal is now higher and the braking better its not hard like a 2023 vehicle . solution ..Change all the flexi brake pipes for modern ones. The 25 year old ones are expanding slightly with the pressure softening the pedal feel.
Works for me .

 

Can you provide a picture of how you plumbed the system?
Can you also provide part numbers?

This seems like an interesting project I would like to try out.

 

Really nice modification, but a related question, NA engines always have a negative manifold pressure unless WOT. Where does a supercharged engine get its vacuum if the manifold is under positive pressure?

 

the supercharged engines only have positive manifold pressure when the throttle is pressed, otherwise all the air goes through the bypass and you have vacuum in the manifold

 

12v UP28 aux boost 20804130.
The various part numbers are below ..diagram of plumbing at bottom.

Manufacturer Part Number
31317445，20914523，20804130
Reference OE/OEM Number
GM 22804112, ACDelco 20914523, VOLVO 20804130, VOLVO 20939309, VOLVO 31317445, VOLVO 31201707, VOLVO 31317530, 009428081, 178-0884, 20997418, 009428-08, 26110FJ00, 20804130, 20939309, 31317530, 20914523, VACUUM PUMP, VACUUM, VAKUUMPUMPE, UNTERDRUCKPUMPE, für, for, VAKUUM PUMPE UNTERDRUCK, transporter unterdruckpumpe, transporter vakuumpumpe
Type
Electric Vacuum Pump
EAN
Does not apply
Other Part Number
20804130, 20939309, 31317530
Warranty
2 Year
MPN
31317445,22804112,20914523
for GM, for Volvo XC 90, for GMC K1500 72, for Chevrolet Swift, for Buick, for Cadillac, for SUBARU FORESTER 15-16.GM 22804112

ACDelco 20914523

VOLVO 20804130

VOLVO 20939309

VOLVO 31317445

VOLVO 31201707

VOLVO 31317530

009428081

178-0884

20997418

009428-08

26110FJ00


RHD version

 

very good!
I think I'll try to add the pump to help my brakes.
Question, where did you source that good looking check valve?
wj

 

UK ebay 2 for $6 ..fantastic item No 195009657632

 

thanks
wj

 

Boosted Brakes Driving Review.

Prior to fitting there was a 3 inch travel in the pedal before you came to spongy brakes land . Now you get 1 inch travel and the brakes are hard ,give you confidence and you get response when you press harder. There is no lag when you have been off the brakes for a while . Good pedal is there instantly. Makes the whole experience much more enjoyable. Bit of foam ..see photo between servo and pump and you cannot even hear it . Fortunately I can right my own engineers report for the insurance company.
Glad so many liked this post its the best mod you can do to improve your XK8.

 

Thanks for the feedback. Definitely going to do this mod.

 

No, no, no.
It is a simple vacuum pump, it boosts nothing!

 

Well OK it allows the brake booster ( Servo) to work properly/produce more brake effort.
Who cares transforms the brakes on the car.

 

While braking the vacuum opening at the brake booster is close, then your pump work?

 

The understanding of that sentence is difficult due to language translation problems but the end result is that the application of increased vacuum improves the brakes to a new level. Try it or forget about it .
I take the high road and you take the low road as they say ....your car do what you want.

 

There could not be any increased vacuum, the brake booster vacuum outlet is closed when your pump begin work………
Language translation problems are possible.

 

You got this wrong. The valve on the Servo body allows air to be sucked out of the Servo and into the inlet manifold. The Pump is only sucking air just like the manifold would but to a higher vacuum level.


Das hast du falsch verstanden. Das Ventil am Servogehäuse ermöglicht das Ansaugen von Luft aus dem Servo in den Einlasskrümmer. Die Pumpe saugt nur Luft an, genau wie der Verteiler, jedoch mit einem höheren Vakuumniveau.

 

Hey Pistnbroke, what gauge and length of wires did you use to splice the pump to the brake light relay on the trunk? How was the connection to the relay done? The relay, if I'm not mistaken, is a square box-looking thing that plugs right into the fusebox. I don't see a way to splice wires to it, how did you do it?

 

I just noticed something, for those of us who have a 4.2 V8, the placement of the throttle cable assembly makes it so the vacuum pump needs to be mounted elsewhere in the engine bay compared to where Pistnbroke did so. I'm glad I haven't bought the pump or started this project yet, because I had not noticed that issue.

 

If you are doing anything with brakes you need to know what you are doing or dont do it .
Your life ...
I dont have a LHD so I dont know exactly but you need the pump somewhere cool and close to the servo. I dont think till you get it you can choose a location. the pipe for mine was 10mm ID. Post a photo of your brake servo box.
The diagram still applies but get the one way valve the right way !!¬!!
Connection method is in the photo below.

use one of the stop lamp fuses with this break out unit.

 

Hey Pistnbroke, here's the picture of my brake booster area that you requested. The way I see it, I would have to mount the pump external to that area, between the shock towers and the bleed hoses.