Dual circuit brakes - What do they do ?

[Nacnud]

I always thought dual circuit brakes meant that a failure at one corner (e.g. hose snapping) would still leave you with brakes on the other circuit allowing the car to stop in a reasonably controlled fashion.

So how come on Saturday, when I had a major failure on the right rear caliper my brake pedal went all the way to the floor resulting in not even mild braking. Several enthusiastic pumps also produced no effect before I turned my attention to other means of slowing the car.

Like I said though - WTF are dual circuit brakes supposed to do for me ?

[jellison]

Hmm - Duncan that is what should happen, can be split diagonall or front to rear (more common I think). Have you had braking system dicked about with since big front brakes - balance bars etc.

My TR still has no split and need to get done over winter in case waht happened to you happens on track.

Still tinking along line I spoke to you about last - get Chim back late this week.

JON

[Nacnud]

Servo and master cylinder wasn't touched in the rebuild. Work was to use braded hoses throughout, and replace the front calipers.
I don't believe the brake circuits were altered at all; but I'll double check.

[Nacnud]

I guess there must be a balance mechanism between the two brake circuits; dunno what or where this is though.
If this balance mechanism failed then could it explain my total brake failure?

[AM400]

The same thing happened to me, pedal straight to the floor but managed to get a bit back with a few pumps of the pedal.Replaced all the hoses for braided ones and a new master cylinder seemed to do the trick, but you are right about the dual circuit thing, it should have left you with front brakes!

Andy.

[shpub]

If you put bigger brake callipers on the front with bigger discs you can suffer from several problems. The first is that you can get pad knock out where the pads knock the pistons back and require more fluid movement to put them back. Caused by movement on the diac by the taper wheel bearing. This means that the pedal goes to the floor but braking should be restored when pumped. The second issue is that the master cylinder is not big enough to move the right amount of fluid and is marginal. If one of the circuits fails, the pedal moves until the internal master cylinder piston spring compresses and starts to move the second remaining piston. Unfortunately, this movement is stopped when the pedal hits its stop and the pedal is on the floor with insufficient pressure to make the front brakes work. This problem can also occur when the master cylinder seals start to go which is why replacing the master cylinder often solves the problem.

The cure is to fit residual pressure valves and a larger bore master cylinder. Exactly what I had to do on the 520.

Steve
www.tvrbooks.co.uk

[Nacnud]

I'm not sure that I've suffered pad knock out as in 9000 miles I've had nothing but a good consistent feel from the brake pedal. However, the master cylinder being badly worn is very possible as it is probably original and I can see the logic of needing a bigger bore master cylinder.

I think I can feel a new master brake cylinder going in very soon....

Cheers for the advice

[Nacnud]

Steve,
Please excuse my ignorance, but what are "residual pressure valve"s and what do they do ?

[pebbledash]

They are like one way valves in the brake Lines. they keep a set pressure in each of the brake lines to Aid braking.

Very simple explination hopefully shpub will supply the Full blow Details soon.

www.wilwood.com/products/master_cylinders/rpv/rpv.asp

edited to add the link.

>> Edited by pebbledash on Monday 11th August 12:25

[shpub]

Correct. Used on cars with drum brakes to maintain the linings in position. Sometimes built into the brake master. The disc version shave a lower pressure and also prevent fluid leaking back with floor mounted pedlas/master cylinders. The residual pressure pushes the pads out so that they stay in contact. AP use springs inside the caliper/piston to do the same thing.

[Nacnud]

Excellent - cheers

[jellison]

Dunc - if in any dobt on braking issue stuff contact the master - Bob Green - can't remember no. in midlands somewhere - will give adice on this type of thye - and what size master cylinder you should have withe brakes as they are now.

[rev-erend]

Duncan,

I had the same thing happen on my SEAC several times -
I drove from Surrey to 'Peninsular' like that - you sure leave a nice gap when you know you might have to pump the brakes to stop.

It seemed to happen about every 10 applications and always worked on the next press.

Peninsular replaced the master cylinder for about £110
for a brand new one.

[Nacnud]

Rev - Hi!
The main failure wasn't the master cylinder; a rear caliper failed big-time and I'm talking to Peninsula about that already. It's why the dual-circuit didn't dual-circuit that I'm puzzled about.

I suspect the master cylinder is original and probably ought to be replaced; plus there are a couple of compelling reasons from Steve H relating to my earlier brake upgrade and hey-presto; I need one new brake master with bigger cylinders.

So if anyone sees a light blue SEAC down Exeter way - you can nod in a knowing way and tell people it's there for some work on the brakes.

[Nacnud]

Incidentally - this isn't my first serious brake problem. I had a Reliant Scimitar for a year and had endless trouble.

One trick it started to do was to lock the brakes on!
They could only be freed by banging the brake pedal hard which was not easy while on the move.

Out of curiosity I dismantled the master cylinder and it wasn't the seals or piston that was worn; it was the cylinder walls! They had such a step worn into them that the piston was being prevented from returning! I guess the good kick gave the piston enough momentum to get over the step.

Mind you - the Scimitar had double the milage of the SEAC.

[rev-erend]

Hey - I always new deep down it would be those
huge new calipers you fitted on the front !!