Split brake systems.

[Lobo]

Bit naughty as non SOHC - forgive me.

Changed out a rear brake line (old Kombi) recently and upon bleeding the brake thought, “bloody hell, how come the pedal goes effortlessly to the floor during this process… as the system is (supposedly) split”

What am I missing pls? The Master cylinder has divided / isolated reservoirs, with each feeding a separate circuit via a common piston / cylinder mechanism - ie front & rear. It seems to me, should a line rupture occur anywhere, all braking will be lost.

Scratching my head …. and expecting to be embarrassed!

[Bryanj]

Even on split system you need to bleed all four

[McCabe-Thiele (Ted)]

As has been said when bleeding the system the pedal will go the floor that is pretty normal.

On many makes of modern vehicles fitted with ABS even when fully bled with no engine running if you maintain pedal pressure continually the pedal will virtually go down to the floor after a few minutes. In addition if your car has a version of brake assist the movement of the brake pedal with speed causes extra braking force to be applied by the brake booster (servo) this only operates when the engine is running.

I have found that if you have a good pedal with no engine running when you start the engine the pedal will go down even further - this usually confirms that the brake pedal booster(servo) is working as it should. A single pump of the pedal often results in a nice solid pedal again.

I had a front brake hose failure on a Carlton many years ago due to it not having had a recall on the front hoses. A balloon shape like an egg appeared on the hose. I was driving towards a roundabout when this happened - dual circuit was sufficient to bring the car to a stop but it was well scary. Brakes pedal was pretty much on the floor.
It felt as though my brakes were pretty useless I would estimate a 75% loss of braking.

[Lobo]

Thanks gents, though still confused. Front discs, rear shoes adjusted, system fully bled & good braking occurs within 1” of pedal movement.
Now, open a brake bleed nipple, and the pedal goes to the floor with little effort (and that slave cylinder obviously ‘bleeds’)

And there’s my confusion… I’d expect appears the ‘other’ circuit of the split system to retain some pedal pressure / stop it going full travel to the floor.

Your last para Ted perhaps addresses it, and suggests the Carlton system was split F/R is opposed to diagonal opposites… given your estimate of 75% loss and no mention of steering upset?

[McCabe-Thiele (Ted)]

You are correct I ended up with what felt like rear discs only. Iirc I tried but failed to follow the brake lines to determine if it was dagonal or F&R split.

I too would expect the pedal to only go down halfway but that has never been the case. I gave up trying to work out the logic a long time ago.

I have also found that a poor brake pedal often is self fixing either from standing over a weekend or after a brief period of use I perceive some sort of automatic air removal must take place.

[K2-K6]

Worth a look https://youtu.be/oVE2pQuMJM0 that shows how a basic tandem cylinder works.

You may have to hit the pedal fast with one circuit open/ compromised to force the emergency capability to function, as in pure panic when you realise the brakes don't work 

Most system bleed through as you've described when carrying out normal bleeding, without having to do anything in the way of adaption. 

You could try stamping on pedal with one port open and piped into a receiver to assess, test functionality.

[Lobo]

Thanks Ted / Nige - problem now solved.

About 3 yrs ago I upgraded the brakes from 4x drums to 2x disc + 2 rear drums. This conversion specifically requires a split system M/C; whose 2 outputs are each tailored to the disc & drum circuits.
And I bloody plumbed them the wrong way around. Doh!

So now, with either Front or rear fluid loss, the pedal goes 80% travel before hardening up. Still gonna be heart stopping I guess… as Ted described.

(the split M/C common to both the Beetle and the Split Screen Kombi for this conversion. Except in the Beetle the primary piston is aft, and in the Kombi - forward. So, whilst I can’t particularly remember, I’m guessing I used Beetle write ups for the instal)

[K2-K6]

Oops!  Simon 

At least you've got to the bottom of it now, hopefully confidence that it's working as intended  for the safety function.

Think most early split mastercylinder were front feed and rear feed discreet system type as already described.

Eighties saw more with four pot calipers front, two piston on one feed and independent, with the other pair in tandem with rear circuit to give front only or four at reduced squeeze on the other circuit.

Austin princess (ADO71)  calipers may have been the first of this orientation and still sought after in competition car circles as they are potent, rigid and fit native inside 14" rims 13" at a push.
Also Mini Metro & MG type vehicle generally used litte four pots which are similarly routinely found on competition cars.

If you wanted a more comprehensive redundancy then something like that may be a consideration  to split the front in same way.

Properly specified and fitted though, there's not often great cause for concern in reliability with decent components though. I use aeroquip for the flexible links if I fit anything in the way of modified system on cars too.

A few round here have had squirrel attack, they go through rubber/fabric type with ease.

[Bryanj]

I liked the Rolls idea of a twin system
2 master cylinders and 2 calipers per wheel

[Lobo]

Never seen both a Kombi and Rolls Royce referred in the same discussion 😂.

My dad bought a Princess - new. Got it home, the boot lock had fallen out, necessitating a new lock / different key. A month later the suspension mounts unglued themselves - back to the dealer much longer. Maybe this due a vibration - the tyres had flats baked into them. A year later the bronze paint had a green copper hue about it. And there was more.
I hardly ever saw my dad cross; my mum had a Datsun Cherry which just ‘went’ - he obviously laboured this fact to Michael Edwardes in a letter …. which had little use. Red Robby probably got to it first & tore it up.