Manky's Pop.

[cunningplan]

Andy/Kev, I know my Mini ran a single circuit, but its very much the same as the Pop. The car weighed 1025kg, had a Jag IRS and Spitfire IFS. the master cylinder was under the floor (Just in front of my feet) the servo was under my seat. I did not use any residual valves at all. The only problem I had was the servo! if I needed to brake hard it would make the car skid, so after a few close shaves I disconnected it and found it was better for it.
As it is still the same chassis that is under my old race car, other than it being 15 inch's longer and I moved the cylinder under the floor next to the seat it still works with no problems.
The new car has only back brakes with the cylinder mounted low down and the brake is fine.
I have used 3/4 cylinders on both cars.
I know you are using a duel circuit, but I cant see why you should have so much of a problem, It could well be the servo and it would not take much the bypass (I use a bit of pipe with a female fitting both ends) it and see if there is a difference.
One of our customers (who owns his own garage) has a push through bleeder. Its got it own cylinder and pot, you connect it the the bleed nipple, open it and pump away slowly. we have borrowed it to bleed clutch cylinders as they can sometime be a big problem and its always worked.

[kapri]

Hi Tony! The problem is that it is not just about feel but the braking percentage is measured at each end based on actual weight, and recorded as primary ( servoed) and reserve ( servo depleted) test and must meet a certain percentage which is higher than MOT requirements .

Because of the nature of the test  tyre height, crankshaft height ,tyre friction and wheelbase all affect the outcome.

I've done similar to you in the past and had to increase m/c size to REDUCE brake efficiency to actually make them stop skidding and be more effective in the real world.

[Manky Monkey]

HELP!!

[terry t]

here is a rough idear of how the brake circut is set up
all the wheel cylinders and pipe work is above the master cylinder and reservoir
there are residual valves fitted to stop any feed back

brake pipes from master cylinder to servo    28"                   highest
twin remote servo                                   24"
rear brake cylinders                                6/8"
front calipers                                        6/8"
brake pipes  to wheels
brake fluid res                                        4"
master cylinder                                      0"                       lowest

when we bleed the brakes we noticed that the front callipers were not releasing

[spanners]

all i can say after looking at this ======OVER ENGINEERED,



stick to the ,BASICS,,

i,m just so sorry that i live to far away  its not a huge problem  but i,m a practical bloke ===crap at putting it into words

[tbone]

The front calipers aren`t releasing?
Sticking caliper pistons or too much pressure?
Can you bypass the residual valves?  where in the line are the valves fitted, i assume straight from the master cylinder?


Is the servo mounted horizontally or at a angle? ..........You should have a slight upward tilt, between 25 & 45 deg, to prevent air getting trapped.

I would bypass the servo initially,
If the problem disappears then suspect either an air lock in the servo or faulty seals (unlikely or you would have noticed a drop in the brake fluid level)
if the problem persists, I would remove & bench bleed the master cylinder, refit and bleed the system (without servo) starting with the wheel furthest away from the master cylinder, then the next furthest, the second closest and finally the closest.

[kapri]

To check if it is once again pushrod length holding front brake line pressure simply check the brakes are still dragging and then loosen the bolts holding the master cylinder to the xmember as this will increase pushrod to m/c clearance.

Logical progression on diagnosing the problem is the key. Check and islolate the function of each piece until you can catagorically see that it MUST be a certain item.

[kapri]

The front calipers aren`t releasing?
Sticking caliper pistons or too much pressure?

Pressure shoudl disappear as soon as pressure on pedal released .So pressure being retained inline by either :-
1) Too long pushrod preventing full return of internals
2)Residual valves
3) M/c having internal residuals as well as external

Can you bypass the residual valves?  where in the line are the valves fitted, i assume straight from the master cylinder?

They are mounted within 22inches of the master cylinder and at the same level .Unable to mount direct to m/c ,my preferred option, as they would protrude through floor .They can be bypassed as the brake system is designed to be modular now .

Is the servo mounted horizontally or at a angle? ..........You should have a slight upward tilt, between 25 & 45 deg, to prevent air getting trapped.

This is a TWIN remote servo, it looks like 2 single servos bolted back to back and mounts horizontally .

I would bypass the servo initially,
If the problem disappears then suspect either an air lock in the servo or faulty seals (unlikely or you would have noticed a drop in the brake fluid level)
if the problem persists, I would remove & bench bleed the master cylinder, refit and bleed the system (without servo) starting with the wheel furthest away from the master cylinder, then the next furthest, the second closest and finally the closest.

Problem with servo could be :-
a) Having to pump fluid about 24 inches uphill
b) Horizontal mounting creating airlocks
c)Head of fluid too much for residuals to cope with.

[terry t]

The front calipers aren`t releasing?
Sticking caliper pistons or too much pressure? [/quote]

Pressure shoudl disappear as soon as pressure on pedal released .So pressure being retained inline by either :-
1) Too long pushrod preventing full return of internals
2)Residual valves
3) M/c having internal residuals as well as external

Even though there sticking.when there is no pressure at the peddle.you have to pump at least twice to get some peddle

[kapri]

Servo in question


http://www.s-v-c.co.uk/product/dual-remote-servo/

Andy, did you bleed the servo via the bleed valves up there ?

[terry t]

Servo in question


http://www.s-v-c.co.uk/product/dual-remote-servo/

Andy, did you bleed the servo via the bleed valves up there ?

no we didn't Kev

[terry t]

here's a bit more info on the servo 

http://www.vwjim.co.uk/ourshop/prod_1182508-Brake-Servo.html

[tbone]

If the caliper pistons can be levered back, would this not eliminate the pushrod?


Would 2psi have that great an effect on the pistons? if the master cyl has integral valves then they should only affect the first 22 inches (upto the valve)?

I still think the servo should be angled, 2 outlets = twice the chance of trapping air, but you now mentioned bleed valves which should eliminate that problem.

[Manky Monkey]

Hi guys.
Thanks for your input. Two things that mean absolutely nothing to me at all -brakes & electrics. Never understood them. That's why I called in outside help in both areas. Terry's doing a crackin' job on the wiring & I trust Kev entirely with the brakes. I really don't know where we go from here, just seem to be going round in circles, but as Kev says, we need to attack it in an orderly, progressive way, eliminating one thing at a time.
To be honest, I'm just about ready to chuck it all in. Been living under the threat of deadlines for months now & all I think about all day is this bloody truck. The fun's gone out of this build completely & it's just become an ordeal. I'm spending more time at Loony's workshop than I am at home.
I should be there now, working on the gearbox tunnel, but it's a gorgeous sunny Sunday morning here & I need a break. I'm taking a day off to go outside & enjoy the sunshine.

[terry t]

have a read of this the part about dual circuit master cylinders
http://www.vwjim.co.uk/news_6694_56041.html

one the setup were using is one pipe from the master cylinder to front brakes outlet on servo.
the 2nd outlet on master cylinder to the rear brakes on servo.
ie two pipe from master cylinder to servo