Front brakes lose grip as boost increases?

[Fred 86 GN]

Since doing the vacuum brake conversion, my GN will not hold much boost at all when power braking, and worse yet the front tires start to roll!

They seem to work well just normally stopping.

Could the vacuum assist be getting pressurized to cause this kind of behavior?

Any ideas?

Thanks,

 

[Nick Micale]

How much boost could you hold before the conversion? How much now?

If the front wheels are turning, the rears must be too.

The vac is only an assist to your leg so if it would even bleed off, it would have no affect on brake holding power if you are pushing hard enough.

Were the brakes themselves serviced, cleaned, need friction material or leaking fluid? Maybe air in the system?

Take your car to a safe place to do a 4 wheel panic stop from 25-30 MPH and see what the skid marks look like. Front and rear even, same length? From this you can begin to see how the brakes are performing. [or not.]

 

[bob86gn]

If the check valve was passing and bleeding boost into the vacuum assist the brakes would go away since the boost would be pushing against your leg. Is that right?

 

[TurboBuickSix]

Sounds right but there is supposed to be the checkvalve in line.Did you put on that black cannister with the elbow fitting on one end and in the right direction?Check it with a vac pump.When this happened to me with my vac brakes it was because I used the cheaper pads in the front,stopping from a high speed was so scary I changed them to the expensive ones the next day and it was a world of difference.To me it sounds like you need to bleed the entire system and go from there.I have the vac setup and was getting 5-6 psi last night(ask the cobra).

 

[Fred 86 GN]

Prior to posting, I did bleed all four brakes. Used a mighty-vac. I did all the bleeding after installing a line-lock. I did not try to bleed the master cylinder at the fittings though. After bleeding, the brake pedal was MUCH firmer than before. Before the bleeding, I could not hold any boost -- the rear tires would spin and/or the front would roll before the needle in the gage moved. Now it gets up to about 4 psi before tires start rolling.

I put new, "high performance graphite impregnated" front brake pads on during the bleeding.

The vacuum conversion was done about two months ago using new (rebuilt) parts from Autozone, so I figure the check valve in the vacuum booster is new since there was a bright pink tag on it stating the vacuum booster warranty would be void if the valve was removed. I plan to test it either tomorrow or Saturday.

I just thought it was odd that the brake pedal "feel" went from pretty firm to spongy/mushy when trying to hold a boosted launch.

Maybe there is some air in the master cylinder from the line-lock installation...but then why would the brakes work good (firm pedal) when not pressurizing the system with boost?