Stopping on a sixpence

[robg]

I am intent on adding a servo to the all drum brakes on my R4 GTL.
To do this I anticipate having to change the master cylinder from a three outlet to a two outlet
used on older models. I assume I can combine the two outlet to one through the servo and split the outlet
front and rear. Is there anything I have overlooked?
Still a relative newcomer to this so any advice would be greatly appreciated.
thanks

 

[Piet]

First of all, I believe you have dual circuit brakes. Three outlets means one to the rear brakes and two to the front brakes. The two outlet cylinders (one to the front, one to the rear) are for much earlier single circuit cars. Some two circuit cars have four outlets, two to front, two to rear. See the thread in the forum below which explains this.

Dual Circuit Brakes, Master Cylinder

Ok I give up. I have 4 outputs on the M/C and seemingly only three places to attach the pipes. 3x M10 and 1x M12. One left wheel, one right wheel and the rear brake junction. Where do the brake pipes go from the master cylinder any one please?

www.renault4.co.uk

https://www.renault4.co.uk/forum/threads/dual-circuit-brakes-master-cylinder.10729/

It is best if you do not combine the two circuits, the whole point is if one fails you will still have brakes on the other circuit-unless you combine them with an old single circuit master cylinder. It is illegal to bypass this.
What you can do (and I have done on my car) is to blank off one of the front brake outlets, so the remaining one goes to a remote booster. The remote booster normally has a single input and a dual output, each of these outputs take the individual front brake pipes that were directly connected to the master cylinder. The rear brake circuit is still connected directly to the master cylinder as is.
You do not really need servo power on the rear wheels. It is usually limited by the brake pressure limiting valve, the load on the rear wheels is normally way less than on the front, with a booster on the rear you would be limiting the pressure most of the time. If you did want servo on the rear you will need a separate booster.

If you were doing a single circuit car, you could use a single large booster but that is not the case here.

 

[Piet]

Addendum- Looking back at this, I fitted a dual circuit (3 output) master cylinder, to turn my early single circuit into a dual circuit car, but because early cars needed an outlet to fit a brake light switch which the later cars don't have, fitting that switch used up the extra outlet hole. You will presumably already have a contact type switch on the brake pedal mount- but if not, that pressure switch uses up one outlet.

 

[André4Renaults]

I wish to council caution regarding your endeavour, for the following reasons:

Brake servo systems were initially only fitted to cars that had disc brakes, when disc brakes were the exception, as opposed to normal, and were usually fitted to higher performance versions of ordinary cars. The servo was needed because disc systems, although more efficient at dissipating the heat generated during braking, and therefore less prone to fading, needed heavier pressures because the area of pad in contact with the disc, as opposed to the area of shoe in contact with the hub, was much less.

Cars fitted with servo-assisted drum brakes tended to have an "all or nothing" feel to them, with normal pressures producing minimal braking, but a little more pressure can result in a dramatic, almost emergency, stop.

It might be a good idea to check the brakes are all properly adjusted and working and also maybe change the fluid, if it's not been recently changed. In addition, different brake shoe materials will give differing braking levels, so perhaps explore this area as well.

Without wishing to criticise your plans I'd consider the above options before embarking on adapting a braking system which Renault's engineers considered perfectly adequate and safe.

 

[robg]

Thank you for the knowledgable and valuable advice. Time, I think, for a rethink perhaps on the way forward.

 

[edwin]

Doesn't a GTL have discs at the front and drums at the back?

 

[JdeW]

Doesn't a GTL have discs at the front and drums at the back?

Front disc breaks;
From 1983: R4 GTL R2118 / F4 R210B and Break R239B / F6 R2370 and F6 covered pick-up 3C2370
From 1986: All Renault 4 models