Brake booster or Tune issue

[VBRED]

When I brake hard from a speed over say 60kph the brakes loose boost (pedal becomes very hard and loses braking) and the engine stalls.
I am thinking that it could be a hole in the booster or at tune problem. What should I be looking for?

[vkberlina]

I would say a hole/cracks in the booster diaphram.

Scott

[VBRED]

I am thinking the same. It seems to be getting worse so the diaphram must be deteriorating. Under easy braking not an issue but under hard braking the cracks must open up creating a major vacuum leaking and the engine stalls. I have a 1" master and VT brakes on the front would one of those double diaphram boosters be the best replacement?

[VK SL 3800]

Double diaphragm will be better then a single unit i would imagine, the VS single type i use works really well. Much nicer feeling brakes then the other VK which is a stock 5ltr berlina.

[VBRED]

I think I am limited to the squat V8 type as the strut bar runs over the booster. I am talking about my VK V8.

[VBRED]

I blocked of the vacuum line from the booster to the manifold. When I brake hard (with no booster) the revs just drop and it stalls. I have tried with a full tank of fuel in case it was the pick up when low. I am at a bit of a loss. The car starts runs idles fine otherwise.

[VK SL 3800]

Float level in the carbie?

[VBRED]

I removed the filter and reconnected the in tank pump so it has plenty of fuel. I also suspect that it must be carbie related. It apears to be getting worse. The carbie is a holley 650 dp.

[VBRED]

Float level in the carbie?

I am now a expert on the Holley 650 cfm double pumper carby after watching all the Youtube educational videos. I have checked the float levels. The rear was spot on the front was a little low so I have adjusted the level. It runs a bit better but still the revs drop very low 400rpm and nearly stalls after heavy braking. I will book it in for a dyno tune next week.

[Not_An_Abba_Fan]

What does it do while stationary when you push the pedal hard?

[VBRED]

Makes no difference to the rpm. Thats why i disconnected the booster. Under braking it still cut out. Thats why i suspect that it is a carby issue or fuel surge.

[Not_An_Abba_Fan]

You could try dropping the float level a bit so that the fuel doesn't over flow into the intake.

[immortality]

I am now a expert on the Holley 650 cfm double pumper carby after watching all the Youtube educational videos. I have checked the float levels. The rear was spot on the front was a little low so I have adjusted the level. It runs a bit better but still the revs drop very low 400rpm and nearly stalls after heavy braking. I will book it in for a dyno tune next week.

That's idling way to low and most likely the reason it's stalling.

To test the brake booster, simply switch the engine off (obviously when stationary somewhere safe like your own drive way). keep your foot on the pedal, if the booster is leaking badly the pedal will want to come up (a leak that bad you should be able to hear). Pump brake pedal (until all vacuum has been depleted, i.e. pedal has gone hard), keep foot on brake pedal and start the car, if the brake booster is functioning then the pedal will drop some. Let the vacuum build up and switch the engine off, let it sit for some time(1/2 - 1hr etc) and then depress the brake pedal, if the booster is leaking then the pedal will be hard, otherwise you will need to pump the pedal a few times to deplete the remaining vacuum in the booster. If it is leaking, check the one way valve first.

I think you possibly have 2 issues, 1. a stuffed one way valve on the brake booster and 2. the carb is out of adjustment