firstly i dont have a carby the vacume line is running right off the back of the plenum just to add my supercharger is switchable and i dont usualy switch it on around town so boost shouldnt affect it

The brake booster should be hooked to a constant vacuum source, not a part time vacuum source [like your plenum] as it is possible that boost pressure can find it's way past the one way valve, particularly if high boost is used, & damage the brake booster which is designed to never see anything higher than atmospheric pressure.

My personal belief is that if you can't guarantee a constant source of vacuum only, then you should configure the brake system so that it does not use a brake booster at all.

If the booster is a must, then one possibility is an altenator with a vacuum pump on the back of it & a small vacuum tank.

Sorry Dodgeman, thats just not true. it should be hooked to the plenum. Just like every other modern engine. Only a millisecond of vaccuum is required to 'recharge' the booster.

On big cam equipped engines being used on nascars, this may be true. I'll tell you from experience that it's false.

I agree the one way valve should be good quality and a quick replacement can be sourced at the wreckers for $2. If you are worried about one failing..just put two valves inline.

Point taken Chris & I really should heve started my post with the words "In my view" as I was expressing an opinion.
You are right, to recharge the vacuum takes only a fraction of a second, however I wonder if the booster has been damaged by boost pressure if the valve does in fact leak a little [not yet demonstrated to actually leak]

Back in the day of triple Webered Valiant six's, these cars used no booster at all because the cam & carbs conspired to make sure that there was no vacuum to speak of in the manifold, so the master cylinder was of a different spec to allow sufficient line pressure without a booster.

Those who had a history of kick starting recalcitrent British 500cc singles had a definate advantage when it came to heavy braking in these unboosted cars.

In the end, RB20 has either a faulty brake booster, or a vacuum supply problem. A check of the actual vacuum in the plenum would be a good start I suspect, then work outward from there.

Quote:

i sent the booster off to get checked out only a couple off weeks ago an it come back fine so i guess that rules that out.
I will check out the vacume pressure tomorrow and get back to you

rb20 man... you have found the problem.

The valve is in backwards. If you pull off the booster vaccuum line and blow down it to the plenum and can't ..it's been installed the wrong way. You might need to get a different one from the wreckers so it'll fit your pipes.

Some things aren't like a jigsaw puzzle. All because it fits, doesn't mean it's correct.


Cheers,
Chris

Quote:
sorry man if you read my quote more carfuly or i explane more clearly it is fitted the right way around

so by blowing through the one way valve from the plenum side it wont flow if you suck from the plenum side it flows
blow from booster side= flow
suck from booster side= no flow

bugger.

You really ought to get decent vaccuum out of the back of the plenum. Certainly without the supercharger switched on. I see you are using a dual piston master cylinder..that's a little unusual in a 1200. conversions.. So it is a Stanza MC? I believe these two piston MCs also do front/back proportioning.

Do you have an inline splitter block (about the size of your hand) installed too? Maybe the two of them are not playing together.

This is looking tricky.

Have you disassembled the MC? If you get them second hand they can oxidise up and then lock up. Or a buildup inside the cylinder can created pedal resistance.

it is a stanza booster and master cylender could it be the lack of splitter that is causing it
pehaps i should go find a wrecked stanza and steel the rest of the brake setup from
it

with the dual chamber MC, the stanza probably doesn't have another bias valve, and will only have a simple splitter for the front lines.

Have you pulled apart the master cylinder before fitting it? Or do you know it was working fine when you got it?