22 replies to this topic

[Muncher]

A few weeks ago I installed my brake servo vacuum reserve canister.


Problem it fixes

This is a fix for symptoms which are a result of a lack of servo pressure, this includes:

- Lack of pedal travel
- Hard pedal feel at the bottom of pedal travel
- Extra effort required to achieve the same amount of braking

The above symptoms occur in particular following sustained wide open throttle acceleration and then immediate braking. Normal braking will usually return with repeated presses of the pedal or with time as the vacuum is replensihed.


What I think the problem is

The basic setup is that the brake servo is powered via a takeoff on the inlet manifold, as the throttle opens the pressure in the manifold drops and sucks air through the servo hose creating a vacuum. A one way valve in the hose in the engine bay prevents air travelling from the engine side to the servo side, thus preserving a volume of vacuum both in the servo and in the hose. At a standstill there is -0.8 bar of vacuum pressure, as the throttle opens this diminishes down to as little as -0.15bar. The servo in theory is able to retain enough vacuum for 3 presses of the brakes before it needs to be "topped up".

What actually happens in my experience (and it looks like a lot of others) is that under hard acceleration insufficient vacuum is retained by the servo for full servo assistance when you get on the brakes quickly afterwards and repeatedly.


How this fixes things

It simply connects in line on the main servo hose, allowing a substantial "reserve" of vacuum to be built up and held behind an inbuilt valve, regardless of what the servo is doing and what is happening with the pressure in the inlet manifold.


I have placed it in the space behind the sill on the driver's side, just where the servo hose passes through. It only requires one cut of the hose and mounting the canister, which I did on the end of the sill section.

I've been out for a long drive tonight and it seems much much better. Not once could I provoke the car into having a dead pedal, rather the pedal actually felt very consistent, even much better than when I didn't appreciate a particular problem. Braking power just doesn't seem to tail off, I like it

I believe Randy has done similar on his race car, also using another one way valve. However I'm inclined to think that isn't necessary as with two valves as it is, a vacuum gauge does not show the vacuum diminishing of its own accord.

I bought mine from a company in the US called Jegs for about $30.

Edited by Muncher, 18 June 2009 - 09:27 PM.

[Retset]

This is the just the result I was hoping for and it sounds like even a muppet like me can fit it. Now, who is going to set up the Group Buy? (No, don't point at me as we are just about to move house and all hell is breaking loose!)

[Winstar]

easiest thing is probably to get someone over hear to manufacture them, they souldn't be dificult or complicated a -0.8 bar isn't a very high load.

[Muncher]

I'm going to draw something up and see how much Pro Alloy would want to make one. I've got some ideas about how to make it easier to make and also mount.

[smithers]

Legend! hopefully this will cure the problem that 'can't happen' according to a couple of tuners I have spoken to. I definitely get this, although I'm still not convinced my ABS unit is shagged as well though... Will be in for a group buy if one is organised, otherwise will go stateside

[Muncher]

Whilst I know this won't change when the ABS kicks in, I have a suspicion that it may help with the "ice mode". If you acknowledge that there is an issue with the vacuum being depleted and stopping the servo from working, then when the ABS unit operates and modulates the brakes on and off very fast it's entirely possible that the modulation ceases to be effective because it has exhausted all of the vacuum in the system. I'll go out and test that tonight but it seems logical that a lack of adequate vacuum is hardly going to help the ABS system to operate when it fires? As with all low volume things made in the UK it's going to be more expensive, but the unit I bought isn't perfect and I think spending a bit extra on something designed for ease of fitment will be a good idea.

[theolodian]

Whilst I know this won't change when the ABS kicks in, I have a suspicion that it may help with the "ice mode".

If you acknowledge that there is an issue with the vacuum being depleted and stopping the servo from working, then when the ABS unit operates and modulates the brakes on and off very fast it's entirely possible that the modulation ceases to be effective because it has exhausted all of the vacuum in the system.

I'll go out and test that tonight but it seems logical that a lack of adequate vacuum is hardly going to help the ABS system to operate when it fires?


As with all low volume things made in the UK it's going to be more expensive, but the unit I bought isn't perfect and I think spending a bit extra on something designed for ease of fitment will be a good idea.

ABS is completely independant from Vacuum supply. Hard pedal can also be ABS blocking you from adding to the line pressures.

[Muncher]

Whilst I know this won't change when the ABS kicks in, I have a suspicion that it may help with the "ice mode".

If you acknowledge that there is an issue with the vacuum being depleted and stopping the servo from working, then when the ABS unit operates and modulates the brakes on and off very fast it's entirely possible that the modulation ceases to be effective because it has exhausted all of the vacuum in the system.

I'll go out and test that tonight but it seems logical that a lack of adequate vacuum is hardly going to help the ABS system to operate when it fires?


As with all low volume things made in the UK it's going to be more expensive, but the unit I bought isn't perfect and I think spending a bit extra on something designed for ease of fitment will be a good idea.

ABS is completely independant from Vacuum supply. Hard pedal can also be ABS blocking you from adding to the line pressures.

Just tried it tonight with the ABS switched back on, feels very good. Sometimes you get the initial hard pedal feel when the ABS goes a bit funny but you can now press through that much more easily. I'd be keen for someone else to try it and see what they think but it seems all good so far.

[cnrandall]

Having been through this process myself I just wanted to add a few things before everyone gets too excited! An extra vacuum reserve does help ensure there is always vacuum pressure available. On my race car I had this exact issue and I used a vacuum reserve (fashioned from an old header tank) and a 3-way check valve setup which cured it. This, however, doesn't cure the 'ice mode' problem which is caused when the ABS system locks out pressure from the master cylinder. Unfortunately, after a fair amount of testing and research, I came to the conclusion that the ABS is very limited and no good on the track. Its an old system that isn't now well supported, cant be re-calibrated, can't cope with different wheel sizes to the standard and also locks line pressure if you achieve more than 1G deceleration which is quite easy to achieve on the track. For track work I would suggest its best to turn it off (I have actually removed it entirely now, along with the servo). If you REALLY want an ABS system for the track the your only real option is to hand a very large cheque over to Bosche motorsport for one of their programmable units.

[slindborg]

I'm going to draw something up and see how much Pro Alloy would want to make one. I've got some ideas about how to make it easier to make and also mount.

far too much no doubt lol

bit of new exhaust pipe, add some plate steel for the ends, put some 10mm barbed fittings on and a couple of inline check valves. No more than £20 imho

if you achieve more than 1G deceleration which is quite easy to achieve on the track.

I was under the impression that you cant break 1G decel without aero (and I mean real aero ) to give extra traction

Edited by slindborg, 20 June 2009 - 09:11 AM.

[Muncher]

I agree chris, my issue was with the ABS turned off.

[theolodian]

I'm going to draw something up and see how much Pro Alloy would want to make one. I've got some ideas about how to make it easier to make and also mount.

far too much no doubt lol

bit of new exhaust pipe, add some plate steel for the ends, put some 10mm barbed fittings on and a couple of inline check valves. No more than £20 imho

How about PVC pipe and plastic caps?

[Muncher]

It would need to be a lot of exhaust pipe to have a capacity of 1 litre+ like mine.

[Retset]

To confirm: I also get the hard pedal with ABS off so knew it wasn't an ABS issue. ABS is just pants really and mine is off.

[theolodian]

To confirm: I also get the hard pedal with ABS off so knew it wasn't an ABS issue. ABS is just pants really and mine is off.

OK, so this can also be an issue on non-boosted engines?

[cnrandall]

I was under the impression that you cant break 1G decel without aero (and I mean real aero ) to give extra traction

Thats one VERY old misconception. If a tyre worked with purely friction then the theoretical maximum would be 1G BUT, it doesn't. A tyre produces grip by making localised chemical bonds with the tarmac and, because of this, the max 1G of acceleration theory doesn't work.

[slindborg]

It would need to be a lot of exhaust pipe to have a capacity of 1 litre+ like mine.

ice cream tub with a good seal on it


hmm the old tossers that used to work at Triumph motor cars (well that says it all ) and Lotus told me wrong then regarding gripppage (they did say with aero you can do more than 1G though so nearly rightish)

[Winstar]

It would need to be a lot of exhaust pipe to have a capacity of 1 litre+ like mine.

ice cream tub with a good seal on it

I was thinking more like one of those rigid tupperware boxes with the rubber seals.

Edited by Winstar, 20 June 2009 - 02:22 PM.

[slindborg]

It would need to be a lot of exhaust pipe to have a capacity of 1 litre+ like mine.

ice cream tub with a good seal on it

I was thinking more like one of those rigid tupperware boxes with the rubber seals.

way to spoil the fun of buying ice cream to mod the car

[dimd00d]

It would need to be a lot of exhaust pipe to have a capacity of 1 litre+ like mine.

ice cream tub with a good seal on it


hmm the old tossers that used to work at Triumph motor cars (well that says it all ) and Lotus told me wrong then regarding gripppage (they did say with aero you can do more than 1G though so nearly rightish)

My father in law is a chemistry professor specializing in tire rubbers, so I can ask him if its of any interest. Although, remembering last time I asked something about tires, its going to be a tire-some experience