Brake Help

[Northern Goat]

I converted my 65 GTO from drums to 4 wheel discs. Got a new master and booster, new distribution block and Wilwood proportioning valve. Front brakes are SSBC and rears from a S15 truck.

I’ve done this conversion on a few cars in the past with no issues. On my 65 however:
I have a good firm pedal (maybe a little too firm)
The brakes seem to work fine for regular stopping
A not quite as good in some cases, but bearable.
Now my problem:
I took the car out today on the back roads got on her good and tried to do some emergency braking. The car didn’t brake well at all, the pedal was down but the car didn’t slowdown quickly at all. Basically took its time.

What did I do wrong? Any suggestions?

[KWIKND]

What is the diameter of the master cylinder?
Dan

[Northern Goat]

I'm confused. Do you mean the booster? If so, it is 10".

[TitoJones]

He means the bore size. 1", 15/16" 7/8" etc. Also what vacuum reading do you get from your manifold at idle, in drive? It may be an assist issue.

Tyler

[Northern Goat]

Thanks for clarifying Tito, I will have to investigatge and get back you guys.

[David Pozzi]

Also what SSBC brakes did you use?
Some of their low end kits use a PBR caliper and skinny C4 12" rotors. By skinny, I mean under 1" wide rotors.