Brakes.....or lack of !

[peteabbott]

Never really been able to get my head around this....
So we have a dual circuit braking system so if one system fails we still have the other, sounds good and safe.
Not a rare problem but my pedal creeps to the floor, quite quickly when she is running and less so when she is not.
The servo assists the braking pressure but the hydraulic side is a sealed system (or two, being a dual circuit)
New master cylinder being fitted today before anyone shouts at me.
The bit I don,t get is that how do both systems fail at the same time, both seals give up together ? Is there a component in the master cylinder that is common to both circuits ?
No fluid loss at all and all callipers and slaves working fine. Up until now the brakes have been perfect.
Have I just lost the plot folks ?

[Cheeky V]

I take it its a dual system where as there are two brake lines that come from the master cylinder instead of the brake balancing thing located underneath just above the steering rack? if its the latter then it could just be that there is air in the system before the split.

Before I replaced one of mine the pedal was super solid and I had to gain 20stone just to use it! after replacing it the servo randomly started working (I didnt realise it wasn't as this is my first bus) and it felt really light, I thought i'd broken something but apparently not! Cheeky V stops on a 6pence now sort of

[bigherb]

Depends on whether it is a Diesel or petrol engine. Diesel will creep down petrols shouldn't.

[peteabbott]

Depends on whether it is a Diesel or petrol engine. Diesel will creep down petrols shouldn't.

Yeah OK thanks bigherb, (thought petrol and diesel brake parts were the same) but I still don,t understand why it is on a dual circuit system it seems quite common that both seals fail at the same time. Surely it is not a case of one master cylinder seal failing then a time after that the other fails ? If only one failed, you would still have quite a good pedal because thats the idea of dual circuit ? Just been scrolling through the site and quite a common question is "what is wrong with my brakes as the pedal goes to the floor "? Two systems, master cylinder with two pistons and seals, two hydraulic lines feeding one front and one rear brake each. Have I got this wrong perhaps, please say if I have.
Oh yes (E D I T) if you had a rear wheel cylinder fail you should have "two corner brakes" working yes ? Dual circuit it is then ?
(Sorry, silver haired old boy with a touch of OCD )

[bigherb]

Diesel and petrol brakes are the same but the way the vacuum for the servo is created is different. Diesels use a pump which if you keep your foot on the brake pedal for long periods will produce enough vacuum to overcome the seals and pull the pedal down. Petrol engines can only create a certain amount of vacuum and can't overcome the seals.
Tandem master cylinders work by one brake circuit piston hydraulically pushing the other circuit piston down if the seal leaks between the two pistons then the you loose both brake circuits. This often happens when the brakes are being bled and the extended piston travel pushes the muck in the master cylinder between the seal and the piston.

[peteabbott]

Correct. Ten out of ten mate. I was not disputing what you said at all, your comments are spot on thankyou.
Just stripped the old cylinder and yes, as you say, one of the seals is split and I now see how it all works. I can see how it fails now with one seal gone. So one seal gone effects both circuits so it is more of a brake efficiency system with dual circuits than a safety one then regarding the master cylinder but if a wheel cylinder went and master cylinder is good then you will still have a brake. Have I got it now ?

[bigherb]

Tandem brakes are safer if one hydraulic circuit fails due to a leak in the line you will still have one brake circuit but not if the seals in the master cylinder is faulty. For a long time some manufactures would still only fit single circuit brakes because the master cylinder was more reliable.

[peteabbott]

Oh I have just seen Ariel Arrow ! Many moons ago when I was an apprentice I saved every penny to buy an Ariel Leader and it was £389 on the road , that was 1967 / 68 Blimey that was a while ago. Appreciate the chat mate. Take care out there.