Manky's Pop.

[kapri]

Tech update.

Stripping and cleaning the calipers cured the disc deflection problem, however it created an issue with the back brakes.

We're using a VW camper master cylinder and this is designed to lose pressure to the front brakes on initial effort. Obviously done as all the weight is at the back on a bus. The problem is that for SVA the front brakes MUST lock first. This is checked both visually and on the rollers.

This bleed off of pressure is achieved by allowing the seal on that part of the m/c to leak fluid back into the reservoir on initial movement ...before blocking the port correctly and creating full pressure.

So we've used front port for back annd back port for front and the front brakes lock before the front...RESULT.

Previously, due to the disc deflection, we had a longer pedal travel which allowed the rear brake to work correctly. Now the front brakes have less travel it appears that the bleedback in that port is not being stopped.

The brake light switch is on the rear line and these operate at 15 psi and it's only coming on at the very end of the pedal stroke.

We are unable to pressure bleed that circuit and, like the front, it will need good old fashioned "pump and hold" bleeding ONCE I've measured and observed the spare Beetle cylinder we have to hand, (in the back of Manky's daily).

I pulled the rear brakes apart and observed the cylinder movement. The m/c is 20.6mm and the slave 20.3 so movement at the pedal should be almost exactly equal to movement at the wheel, ( Pascals Law of hydraulics), but it is very slow to operate indicating lack of pressurisation in the m/c itself.

At the same time as this is happening the front brakes are locked on after only around 40mm pedal travel, (2nd pump).

With the first pump the pedal travels to the floor whilst still operating the front brakes fine, exactly as it would do in the event of a rear section brake line pressure failure.

More in next post.

[terry t]

Kev. what i cant under stand is we had good pressure on the back brakes. when i wired up the brake light. i pushed the brake peddle down a bit by hand to test the lights

[kapri]

Sizing of m/c

Back axle is sherpa van. Standard Sherpa set up is m/c 22.2 , front calipers 57mm and rear slaves 20.3mm.

M/c used is a 20.6 VW Camper non servo    m/c is 20.6  , front calipers 54mm and rear salves 23.8mm


Pop features                                           m/c 20.6  , front calipers 54mm and\ rear slaves 20.3mm

From this you can deduce that a Bus has more braking effort on the rear due to the larger slaves, not something we need on a pick up.

The Sherpa use a larger m/c due to the higher volume requirements of the 57mm calipers but, due to the larger bore, this is delivered at lower line pressure.

The camper m/c delivers less volume per mm of stroke but does it at a high pressure.

The reduction in rear slave bore by using Sherpa size bore, ( 20.3), as opposed to Bus, ( 23., means the m/c will move less and this may be why we cannot, (currently), bleed it out as it's not going past the 'bleedback ' part of it's stroke. I have several solutions to this to try after I've taken measurements.

If this were a normal m/c without the 'bleedback' proportioning then all would be fine and dandy right now.

Swapping lines front to rear will make no difference except the rear brakes will lock first, so failing the SVA.

[kapri]

Kev. what i cant under stand is we had good pressure on the back brakes. when i wired up the brake light. i pushed the brake peddle down a bit by hand to test the lights

It's because we have removed lost motion from the front calipers Terry so the rear m/c bore isn't travelling as far as it was.

[terry t]

Kev. it you were to put the pressure gauges on the brake lines.one on each off the front brake pipes. and one on the rear on the tee piece on the axle. bleed the air out then test to see what pressure we got just on the master cylinder and pipes.

[kapri]

Did the front lines Terry, also through the bleed nipples on the caliper and we were making around 500-700, (multiplying effect of servo internals). Impossible to get an exact reading due to trapped air in the test gauges. We've run ridiculous amounts of fluid through the pressure bleeder and there's no air in the lines. However, when we started yesterday it was impossible to get any sort of pedal using the pressure bleeder, on either line, and it was only traditional methods that worked.

I'm loathe to disturb the front system at the mo !

Though I agree that testing at the rear T piece will add another piece to the jigsaw. We were going to do that yesterday but ran out of time, and in my case, strength.

I've only had a few hours sleep as I've been up running figures and checking specs. Going to grab a few hours kip and then shoot up to the workshop for when Andy arrives.

[dobber]

this is doing my head in and im nowt to do with it, good luck today fellas. if i ever have major probs i would like  blokes like you on board.

[kapri]

Luckily I did a lot of training courses with my old firm with the actual manufacturers of components ,and being into motors, I made the most of them

[kapri]

Pascal's Law, basis of braking systems.

http://www.edgeroamer.com/sweethaven/mechanics/hydraulics01/default.asp?iNum=0102

[Manky Monkey]

I gave up trying to understand all this several weeks ago. I'm just there to press the brake pedal & top up the fluid when I'm told to. 
Right, as far as I know, the current situation is:
There's definitely no air anywhere in the system or any leaks that we can see.
We've got really good front brakes. They're locked on solid with very little pedal travel or effort-almost just the weight of my foot on the pedal. Pushing the pedal down hard actually flexes the 6mm mounting plates.
We've got a good handbrake that locks the rear wheels up solidly on 3 clicks of the lever, showing that the brake shoes are correctly adjusted.
We can lock up the rear wheels, but the brake cylinders aren't moving early enough, meaning the pedal's almost on the floor before they're locked.
We spent a lot of time this afternoon fine tuning the pushrod that connects the pedal to the master cylinder. It seems to be very critical -just a fraction of a turn on the adjustable rod end makes the difference between the front brakes being off or locked on solidly.
So. The problem appears to be that there's too much pedal travel between the front brakes coming on & the rears. We could alter the pedal, but it'd mean a major re-design of the driver's side of the cab & at this stage, isn't really an option.
Because it's a dual circuit system, there are 2 pistons inside the master cylinder, one behind the other with a spring between them. Pushing the pedal moves the first one which operates the front brakes. This in turn pushes the second one, operating the back brakes. The gap between the pistons dictates the amount of pedal travel between the front & back brakes operating. We need to reduce this gap, so that the rear brakes work sooner.
The voice in my head shouts That's wrong. It's a bodge. You shouldn't need to meddle with a factory built unit like that to make it work. It'll make it unsafe, etc, etc. But we haven't got the same components the cylinder was designed to work with. We've proved that everything else is working correctly, so the simplest, quickest way to reduce the delay between front & rear brake operation is to put a spacer in the cylinder, in front of the second piston. That'll mean the first piston will contact the second one earlier, operating the rear brakes sooner.
2 days to measure it, make it, fit it & test it. Oh, & do all the other jobs we need to do before the test.   
 

[spanners]

i,ve stayed out of this

but its so simple  its over engineered to to bigger mix of parts,,,,






there ive said  it  ,, been thinkin it for weeks ,  shoot me if you like,,


just wish i was close by ,,, it wouldnt have been so much of a ,,,,problem,,,,

[kapri]

Had I been involved from the start I would have selected different components, I wasn't so it hasn't.

The chassis and interior is more of less built around the bits being used. There is not enough ground clearance to use a bulkhead type servo under the floor , the path of the steering column , and closeness of engine to bulkhead, precludes twin side by side single cylinders, same with normal bulkhead mount. Cant reverse mounts twins ( like on my 27T) either.

The option I would have taken would have been a bulkhead style serv/mc mounted across the car under the dash with a bellcrank mechanism from a suspended pedal , done several that way where space was tight. Again would have required major reworking that was not possible in the time frame to the initial SVA test.

Likewise,time frame , and budget ,to this retest made the decision to work with fittings we have available.

It isn't just a question of getting brakes, we've had them several times that were fine driving around the yard, they need to pass a stricter than MOT spec and the equipment shows up stuff that would never show in normal driving or MOTs.

[Manky Monkey]

When we started building ths project, we had no idea about the maths involved in getting everything working in harmony. To be honest, we didn't really think about it -we need rear brakes ..use the drums that came with the back axle. We need front brakes ..use Triumph discs cos they can be made to fit fairly easily & someone had some we could have at the time. We need a master cylinder ..use a VW Beetle one cos there were several V Dubs in the yard at the time. We need a pedal ..adapt the original Pop one.
We've never built an entire car from scratch before & we've learnt a lot along the way -usually by doing it wrong the first time & having to re-do it, often several times, until we got it right. In hindsight, we probably couldn't have picked a worse mix of components, but time -& space, are tight & we don't have the luxury of pulling it all out & starting again.
Loony's Dad, Dave, came down to the workshop today & fired up his lathe to machine a nylon spacer to fit between the 2 cylinder pistons. 4.5mm thick with a locating nib on one side & a corresponding hole on the other. He then took 4.5mm off the pin on the back of the second piston, which acts as a stop to limit pedal travel. So we should have the same amount of travel in the cylinder, but the rear brakes should come on sooner. With a pedal ratio of 5:1, that should reduce the pedal travel between the front & rear brakes being applied by 22mm. 

[Manky Monkey]

Just had a phone call from Scottie, asking if we could load the truck on Tuesday morning rather than Monday afternoon as it's his missus, Gina's birthday tomorrow. That means, to be at the test station in Southampton by 7, to warm the truck up for the test at 8, we have to meet at the workshop at 5.30 on Tuesday morning!

[cunningplan]

Just had a phone call from Scottie, asking if we could load the truck on Tuesday morning rather than Monday afternoon as it's his missus, Gina's birthday tomorrow. That means, to be at the test station in Southampton by 7, to warm the truck up for the test at 8, we have to meet at the workshop at 5.30 on Tuesday morning!

so being a postie, your going to have a lay in 
Good luck