Topic

We would need to know which cars had ABS and the bias valve.
Would cars with the standard brakes and ABS be different than cars with APs and ABS?

According to RAVE (8th Edition), cars with ABS have a Brake Pressure Proportioning Valve.

My 1998 F has got ABS. I'll have a look in the morning to see if it has the valve fitted. From what I remember, I'm pretty sure it has.

Robin

I can understand a bias valve if 240mm discs were fitted all around even with ABS.

FWIW..my LE500 (with AP brakes) has ABS and no brake proportioning valve...at least not in the front boot...the bracket for one is there but the pipes show no signs of being mucked about so I would guess one was never fitted..

As I thought, my 1998 VVC F has ABS and the bias valve, with the standard size brake discs.





Robin

My 1996 MGF with ABS also has that proportioning valve - but of course was also fitted with 240mm brakes all round.

Very interested to see that at some point MG Rover deleted the proportioning valve?

I'm wondering if they removed the valve for cars equipped with APs and ABS as standard items?

That's a good question! Tipper's TF120, with standard 240mm brakes all round and ABS does not have the proportioning valve.

Therefore I wonder whether, on TFs at least:
ABS = no proportioning valve
No ABS = proportioning valve fitted

Anyone have a TF that does not fit this rule?

The TF ABS rule is correct: Nick (Austin garages) posted the answer on the MGFRegister forum: Bosch 5.3 ABS has an electronically controlled "Brake Force Distribution" valve that renders the bias valve superfluous.

I am still interested in your observation David that your car effectively does not need the valve at all with APs fitted at the front. But unfortunately I don't think it is a good idea for those with earlier Bosch 5.0 ABS to simply delete the bias valve.

I would tend to agree with that analysis. There are a number of possible combinations of brake disc size and ABS fitment and type (5.0;5.3).
Each situation would need to be individually assessed.

All I can state categorically is that non ABS cars that are upgraded to APs will benefit from a different bias.
Whether that is by fitting an adjustable valve or simply removing the standard proportioning valve could only be decided on a case by case basis.

Retrofitting APs to a car that already has ABS could be complicated.

We have to accept that it seems de rigeur to swap the 240s to 304s with no regard to balance. At least prompting debate about front rear balance after this massive change has to be good.

What impact does fitting APs and 304mm discs have on the MOT? And for that matter adding an adjustable bias valve?

I know that there are tolerances for left and right on front and rear but are there upper limits for braking force or is it just a lower limt at the front and at the back? Also, are there figures for front / rear balance?

You'd expect that as long as the braking force at each end is above the minimum and within tolerance left and right, the car would pass but I wonder if there is something about front / rear as well.

Just interested really.

Robin

The main impact is that you will have better braking figures.
There is a minimum percentage of the car's weight that needs to be achieved in terms of brake force. I think it's 50%. In other words if the car weighs 1260 kgs, then you need at least 630kgF of brakes.

If your 240mm setup is in good condition then you can expect to get (2x220)+(2x170) = 780kgf. Just over 60%
If they're in poor repair you will struggle to achieve 50%.

With APs you probably achieve minimum 50% with the front brakes themselves. My results suggest a total brake force around 1100kg
So a brake efficiency of over 85%.

As you say there is a balance check side to side but I don't know if there is an acceptable front/rear balance figure but I will find out from my mate.