Reducing servo assistance using Vacuum regulator

[NDNDNDND]

I've an Alfa Giulietta. It's a nice car, but like most modern cars, it comes with a myriad of dynamic flaws.

Most can be driven around but the most egregious is the brake servo assist, which has been tuned so that the brake pedal is lighter than the throttle. As a result the brakes are very snatchy and difficult to modulate.I tried driving it in my socks the other day, and braking was performed by flexing my big toe rather than using my foot and it was still difficult to finely control the brakes. It's very silly.

One way I could reduce the servo assist to less-stupid levels would be to introduce a controlled bleed into the vacuum supply to the servo.

Here's a product that performs a similar task on ND MX-5's albeit at considerable cost:
https://www.miataturbo.net/race-prep-75/miataspeed...
According to that thread, the valve is a Konganei NVR.

I'm researching using this product:
https://www.rowse-pneumatics.co.uk/airtac-GVR20008...
...spliced into the vacuum line to give the same effect. The addition of a gauge should make the adjustment more predictable. Vacuum is provided by a vacuum pump, so intake vacuum cannot be affected. There is, however, a connector to the servo check valve on the (massive) servo itself, so I don't know if reducing the vacuum might throw the ECU into conniption.

Has anyone any thoughts or advice on using this method to reduce servo assist?

[stevieturbo]

indeed....this is very silly.

[GreenV8S]

I can sympathise with your position on stupidly sensitive brakes. I had one hire car that was so bad it was almost impossible to use the brakes below about 10 mph without doing an emergency stop. However, mucking with the vacuum supply to the servo is IMO not a sensible way to resolve that. I think it's either a design fault, or perhaps the valves in the servo being mis-adjusted so the assistance is too aggressive.

[Maxdecel]

Did this phenomenon start after you fitted new pads ?

[NDNDNDND]

It was diagnosed with deteriorated wishbone bushes at its last MOT and these were replaced. This did eliminate some of the snatch, but the characteristics of the brakes remained largely unchanged. Brake fluid was changed at the same time. Plenty of life left in the pads, but the rotors are a little lipped and will need changing with the next set of pads.

Would there be any way to diagnose a mal-adjusted valve in the servo?

[Maxdecel]

I've never heard of an over performing servo before ! How was this diagnosed?
Pad life is irrelevant, best guess is the friction material has a high friction coefficient (Mu) competition compound probably and far exceeds the original design parameters.
You need to establish what pads are fitted to the front axle.
This article explains the need to select the correct material quite well, it's about Mini's but friction is friction.
https://www.calverst.com/technical-info/brakes-pad..."Too much friction for any given application will over-come the tyres ability to grip the road (traction)." - If I'm not mistaken this is the inevitable outcome in your case if you press the pedal more ?

[stevieturbo]

Maxdecel said:

I've never heard of an over performing servo before ! How was this diagnosed?
Pad life is irrelevant, best guess is the friction material has a high friction coefficient (Mu) competition compound probably and far exceeds the original design parameters.
You need to establish what pads are fitted to the front axle.
This article explains the need to select the correct material quite well, it's about Mini's but friction is friction.
https://www.calverst.com/technical-info/brakes-pad..."Too much friction for any given application will over-come the tyres ability to grip the road (traction)." - If I'm not mistaken this is the inevitable outcome in your case if you press the pedal more ?

Some cars may be a little more sensitive to the pedal than others. But generally you just don't press the pedal as hard then. Pretty easy and free solution.

I would class my dads golf as pretty sensitive, but even my dad with huge boots because of arthritis ridden feet, and multiple joint replacements....has no issue driving it and braking smoothly.

If someone is really struggling that much with their brakes, perhaps driving lessons ? Or maybe someone has fitted some very weird pads or something

[NDNDNDND]

Maxdecel said:

I've never heard of an over performing servo before ! How was this diagnosed?
Pad life is irrelevant, best guess is the friction material has a high friction coefficient (Mu) competition compound probably and far exceeds the original design parameters.
You need to establish what pads are fitted to the front axle.
This article explains the need to select the correct material quite well, it's about Mini's but friction is friction.
https://www.calverst.com/technical-info/brakes-pad..."Too much friction for any given application will over-come the tyres ability to grip the road (traction)." - If I'm not mistaken this is the inevitable outcome in your case if you press the pedal more ?

No, over-performance has not been diagnosed - at least not as a failure mode, more as design feature.

I would be very surprised if anything other than extremely ordinary brake pads have been fitted. I'm sure you're aware that high-performance pads are usually considerably less sensitive when cold, and require heat before they offer greater performance. I have even considered fitting trackday/race pads (EBC yellow/red) as a way of dulling the brake response, however I suspect the performance may become more unrpredictable as a result.

[NDNDNDND]

stevieturbo said:

Some cars may be a little more sensitive to the pedal than others. But generally you just don't press the pedal as hard then. Pretty easy and free solution.

I would class my dads golf as pretty sensitive, but even my dad with huge boots because of arthritis ridden feet, and multiple joint replacements....has no issue driving it and braking smoothly.

If someone is really struggling that much with their brakes, perhaps driving lessons ? Or maybe someone has fitted some very weird pads or something

Ah, the good old ad-hom. Thanks.

[Maxdecel]

NDNDNDND said:

No, over-performance has not been diagnosed - at least not as a failure mode, more as design feature.

I would be very surprised if anything other than extremely ordinary brake pads have been fitted. I'm sure you're aware that high-performance pads are usually considerably less sensitive when cold, and require heat before they offer greater performance. I have even considered fitting trackday/race pads (EBC yellow/red) as a way of dulling the brake response, however I suspect the performance may become more unrpredictable as a result.

"I'm sure you're aware that high-performance pads are usually considerably less sensitive when cold,"

Old wives tale, use to be the case maybe last century. Modern high µ materials start higher than OE and generally get higher with increasing temp.
As for a design feature ? When designing braking systems, legislation stipulates certain requirements in performance usually based around 500N pedal effort tested against the vehicle at GVW and various other conditions. Assuming your car is as it left the production line then it's very difficult to believe it's a "Design feature"
Anyway you are confident OE/R90 approved pads are fitted so I have no further suggestions, best of luck with a fix.

[GreenV8S]

stevieturbo said:

Some cars may be a little more sensitive to the pedal than others. But generally you just don't press the pedal as hard then. Pretty easy and free solution.

The hire car I mentioned was almost undriveable at low speed although no problem once it was moving. It was almost impossible to apply the brakes at walking pace without them grabbing. I suppose it may have been some sort of brake assistance that thought it was helping me to do an emergency stop.