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bobbybrown
08-10-2008, 07:42 AM
I never realised we didn't have a forum for brakes! :oops2:
My question is, what is the difference between single circuit and double circuit brakes and how do I tell the difference to see what I have?
Hopefully I'll be buying all new brakes on Friday/Saturday so I need to know exactly what I need before I go off buying them.
Also, where is the master cylinder normally located as I tried bleeding the brakes the other day and nothing happened, nothing came out. I removed the bleed nipple entirely and a big puff of fluid came out, put the nipple back in and nothing no matter how many times the pedal was pumped.

Thanks in advance :)

Alzax3
08-10-2008, 08:22 AM
Single circuit means that both the front and back brakes are all connected, so if a pipe bursts anywhere, you lose all braking.....Dual circuit seperates the front from the back, so you are left with braking at one end or the other if a pipe gives up. I never thought to check, (despite having renewed the metal pipes) - just assumed that being an ancient design, the Beetle will have single circuit brakes, it certainly doesn't have any sort of compensator valve at the back.

The master cylinder is probably mounted on the back of your pedal cluster (hidden away under the glass fibre, you can see it if you take the drivers side wheel off), which is why there will be a remote filler pot on the passenger side under the bonnet.

bobbybrown
08-10-2008, 08:27 AM
And what does the problem sound like with the bleeding?
I'm hoping its not the master cylinder as it sounds like a PITA to get to.
If I'm lucky this week, a miracle will happen which will leave the car in MOT'able condition within a couple of weeks :tongue:

Alzax3
08-10-2008, 08:32 AM
Well - if you got a puff of fluid out, something has been pressurising the system, (unless you've pushed the pistons back into the calipers (or is it drums all round?) Do you know that the master cylinder is actually moving? It's one of the things that can sieze/rust solid when a car has been laid up for years........

bobbybrown
08-10-2008, 08:34 AM
I have no idea if the master cylinder is working, and yes, for the time being its drums all round :thumbdown:
The shoes and cylinders are brand new, well they were when they were fitted so I will be leaving them in place if I can get them to bleed properly.
I do need new adjusters though as the star wheels don't work properly on one drum, well on the one I've checked.
One star wheel is ok but the other one doesn't move at all so that will need replacing.

I don't want to buy absolutely everything for the brakes if I can help it, some of the components are in perfect working order, such as the shoes and hopefully cylinders.
The drums look near enough new too, but one is broken where I smacked it with a hammer so both fronts may be changed.
Does anyone know if the condition of the back plates is important for the MOT?
The front passenger back plate has a couple of holes in it that I can get my little finger through due to rust.
Just want to know if I definately need to replace it before the MOT or not.

Alzax3
08-10-2008, 08:44 AM
I suppose if the master cylinder had siezed 'down' then you might get the 'puff' effect and then no further joy...it sounds depressingly like it needs some investigation, and may be better in the long run to replace it rather than mess with a rebuild kit (which won't help if the cylinder is pitted) Job to say about the back plates, I've seen some direly rusted ones pass the MOT, mostly with brakes I think you need to ask yourself whether you feel safe trusting your life to them, and if there's any doubt trust yourself rather than what a tester might think :wink:

bobbybrown
08-10-2008, 08:46 AM
Ok then, lets hope the brakes are single circuit then as it means the master cylinder is only £20, verses £60 for a double circuit one :laugh:

Any obvious tell tales if I look under the car what type it is?
Or if I can somehow see the master cylinder.

Thinking about it, wouldn't I have seen the master cylinder when the dash was out if it was where you said it would be?
Hmmmm.

Alzax3
08-10-2008, 08:52 AM
Only with X-ray vision, as it's on the other side of the bulkhead!

bobbybrown
08-10-2008, 08:54 AM
But I do have x-ray vision and I still didn't see it!
In all honesty, I think I know where it is after thinking about it, when you said other side of the bulk head it hit me that when I was bolting my pump the the front of the tub I think I saw it and took no notice what so ever.
If I'm right, then its going to be a total headache to change or even get to to check it out!

How many pipes come off the single and double circuit cylinders?

Alzax3
08-10-2008, 08:58 AM
Okay, I looked it up and it could be dual circuit (in fact, so could mine - I said I never bothered to check!) this link: http://www.vwheritage.com/vwh/VW-Beetle-Parts/Brakes/Brake-System/?templateID=vwh
shows the two sorts of master cylinders - so sticking your head under the wheel and looking at what's there (or I think checking whether you've got one low pressure pipe or two running from the reservoir) should tell you how much it's likely to cost :annoyed:

bobbybrown
08-10-2008, 09:00 AM
Ok, excellent.
I'll stick my head under and see what I find, if I can get my phone in there I'll take a picture and post it up. Won't be until tonight, maybe tomorrow though as even though I will be at the car tonight, it will be for other business and I doubt I will have time left afterwards to even look at the car.
Trying to find time to work on it in between going to work at 3am and sleeping is pretty much impossible!

Going by your picture, if its a short cylinder it should be single, and a long one should be double, or something like that. (as I doubt I'll be able to see how many pipes are coming off it in a dark garage)

JemP
08-10-2008, 09:32 AM
Mines also double circuit, two pipes from reservoir into cylinder, and seperate pipes out to brakes. Also two connectors for the brake light switch it seems, all works anyway.
I'd start by doing a proper bleeding job all round the car if you can just keep running new brake fluid through all of the pipes till you're happy that none of the old stuff (which will be heavy with water) is left, I usually buy new nipples as they get ripped up so easily, and they're cheap. That way you can check each individual wheel set up, if one brake isn't working at the wheel, its time to investigate. Also remember that on the drum brakes, there is a little adjuster wheel accessible through the back plate, for easing off to get the brake drums off, make sure this is adjusted for the new shoes.
I needed new shoes, one brake cylinder, 4 nipples, two new brake calipers for the front with new shoes, and a damn good clean all round and my brakes worked for the first time in 8 years at least!
I bought one of those one man bleeding kits, the type that attaches to the spare wheel, I found it brilliant.

bobbybrown
08-10-2008, 09:38 AM
You can also use a piece of washer tubing and a syringe, the plastic type found in hospitals, has the same effect :tongue:

This has been sat for about 6 years, so I'll definately try your suggestions before writing off the master cylinder altogether.

Big Birds Car
08-10-2008, 09:52 AM
Dan check your email.

bobbybrown
08-10-2008, 09:56 AM
Got it Paul, thanks I'll try it :)

JemP
08-10-2008, 10:42 AM
You can also use a piece of washer tubing and a syringe, the plastic type found in hospitals, has the same effect :tongue:

This has been sat for about 6 years, so I'll definately try your suggestions before writing off the master cylinder altogether.


Can you? how does that work?

bobbybrown
08-10-2008, 11:14 AM
Put the washer tube over the bleed nipple and put the syringe on the other end, pull the syringe open and it will draw the fluid out, when the fluid starts to come out it will keep coming and you can see in the tube how much air is in the fluid so you can stop it and tighten the nipple back up easily.

Ben
08-10-2008, 09:50 PM
Hi bobbybrown

Try pulling up the brake pedal with your toe when you have a dry master cylinder.
That posted add some pressure to the reservoir by an airline or foot pump.

Its hard to bleed a dry beetle brake system with just one man. When I was trying to bleed the brakes on my first Nova, I had my brother help me to pump the pedal. Then I was just using a piece of pipe into a glass jar immersed in brake fluid. The instructions I was giving him did end up in a fight :jerry:

Now I do it on my own with a piece of rubber hose blanked off with a split in it to let the air escape and not the fluid. :clapping:

Ben

Spacenut
09-10-2008, 01:15 PM
The dual-circuit Beetle master cylinder has two sets of seals on the same pushrod. This creates one compressible reservoir of fluid at the end of the cylinder, and another one further up. To decouple them the pushrod shaft is split and spring loaded so that the fluid in one reservoir can be compressed independently from the other. One is connected to the front brakes and one to the rear. This is the same set-up as a balance bar tandem master cylinder set-up, and as Alex says, allows either the front or rear brakes to continue working if the other circuit fails.

I believe the "proportioning" effect Alex was alluding to is provided by the decoupling spring in the master cylinder - if it works the way I described (I'm pretty sure I've had a Beetle cylinder apart before, but it is a common generic design), the reservoir nearest the pedal will provide the highest pressure, as the one at the end of the cylinder will limit first, although the decoupling spring will try and equalise the pressure between the two. The lower pressure output goes to the rear brakes to compensate for weight transfer under braking.

Incidentally, the circuits don't have to be split front/rear - diagonal braking circuits operate on one front wheel and diagonally-opposed back wheel, which retains the same brake balance (bias), but is only half as effective as a full system.

If the front circuit fails in a Beetle (or bias-bar equipped car, like Green Machine) you have to be careful when braking as you could spin the car, particularly in the wet. Then again, the same could happen if the rear circuit failed. Bottom line is be very careful if brake failure is suspected!

I just thought - if the master cylinder is a dual circuit, there will be two hydraulic pressure switches, would there not? Just in case one circuit fails, the brake lights will still work - I think that's how VW did it.

So you could just look to see if you have a pair of hydraulic brake light switches...

...And if you do, you could wire them up with some simple logic gates so that if one pressure switch works but the other doesn't it lights an indicator lamp (an logical AND gate should do it)

By the way, I use a Gunsons pressure bleeding kit - it connects to a spare tyre, which pressurises a remote brake fluid reservoir, which then feeds the fluid under pressure into the brake fluid reservoir via a special adapter cap, so there is always fluid under pressure in the system - just open the bleed nipples and watch all the air come out.

Lauren

Alzax3
09-10-2008, 08:03 PM
I'd wondered exactly what the point of the second brake light switch on the master cylinder was - Thanks Lauren! The original builder hadn't connected it, and since the brake lights worked, I didn't worry about it! I guess a pair of piggy-back wires between the two switches would rectify the omission?

bobbybrown
09-10-2008, 08:11 PM
Yep, should do :)

Alzax3
09-10-2008, 08:23 PM
Will do then! So is your dual or single circuit, Dan?

bobbybrown
09-10-2008, 08:39 PM
I still haven't had chance to check, see my your cars thread (had to rip the engine out tonight) but after a long phone conversation with Ben earlier this afternoon, I'm heavily leaning towards dual circuit.

A quick call to ask if he had received something I sent him recently turned in to over 20 minutes of Nova talk!
Ben is like the techical support helpline for Nova related queries for me recently :laugh:

I'll catch up on this thread tomorrow, I'm off to bed as I have to be up for work at 2am and I've had 1 hour of sleep today since getting home at 8am!