Picked up this gem this weekend:

https://youtu.be/-MjDbxlCEcA


JS29:
Dodge Challenger
Special, R/T
2 Door Sports Hardtop

U0E: 440 375HP 1-4BBL 8 CYL
1970
Los Angeles, CA, USA

104265:   Sequence number

E86:    440 cid 4 barrel V8 (High Performance) 375hp
D21:    4 Speed Manual Transmission
FC7:    Plum Crazy Metallic Exterior Color
HRX:    Trim - High, Leather Bucket Seats, Black
000:    Full Door Panels
912:    Build Date: September 12
041816:   Order number

V1X:    Full Vinyl Top, Black
A33:    Track Pak w/3.54 ratio
A62:    Rallye Instrument Cluster Package
C16:    Console w/Woodgrain Panel
C26:    Consolette w/Formed Headlining
C55:    Bucket Seats

J25:    3 Speed Wipers
J45:    Hood Tie Down Pins
M21:    Roof drip rail moldings
M81:    Front Bumper Guards
N41:    Dual Exhaust
N42:    Chrome Exhaust Tips

N85:    Tachometer
R22:    AM Radio with 8 Track  (10 Watts)
V1X:    Unknown
V6W:    Longitudinal Stripes, White
26:    26in Radiator

END:    End of Sales Codes

That sounds possible except the whole kit was purpose supplied for the Challenger. Unfortunately I bought it over 3 years ago and the US supplier doesn't deal with tech queries (doesn't answer Emails and I'm in UK). Perhaps its just me and I'll get used to it as I drive it more.

I think I understand what you are saying; I do not it has anything to do with you E brake, it has to do with your MC bore size, I am think it might be on the large side.

All new system with proportioning valve. I have checked that it is centred. Wheels do not lock under heavy braking (at least so far)- it's just that I have to push hard - it feels almost as if I've lost the boost, yet under lighter braking it's perfect. I don't have excessive pedal travel, or a spongy pedal and my pedal ratio is 4-5. I wonder if the E brake system in the rear calipers is affecting anything although I have adjusted it carefully as specified and the E brake works well.

Alot of factors here but first off, a non ABS car will lock up the tires if the brakes are slammed unlike ABS which will pulse (releasing and applying the brakes incrementally). That said, in your case I am not sure what you are expecting or what the car is actually doing. If the brakes are locking up and the tires are sliding, thats what an old school brake system will do. Ideally the front lock before the rear, but they will all lock nonetheless.

Have you changed the entire brake system (MC, proportioning valve, metering valve, etc.)?