If a person is going to used non-factory (aftermarket or Mopar Performance) small block valve covers or going big block – you will have to either remove the power brake booster OR go with a smaller diameter brake booster.
T
Chrysler changed from a 4-bolt cast iron brake master cylinder to an aluminum 2-bolt brake master cylinder in 1978 (mid-year – I think, but it might have been the beginning of model year).
Most everything is different, including the holes in the firewall between the 4-bolt (cast iron) and 2-bolt (aluminum) master cylinder versions.
The differences between the (early) 76-77 (or early '78) and (late) '78 to 89 FMJ master cylinder's (and boosters) are:
Firewall holes, Brake booster, Master cylinder, under-dash bracket and the manual brake firewall stiffener plate. This is going by memory so there might be more.
The aluminum brake master cylinder is much preferred over the cast iron version, for several reasons.
The holes in firewall or under-dash bracket are not hard to make (if needed).
This is a late '78-89 under-dash bracket. There are 6 holes where it goes through the firewall. The rust colored is automatic and Black one is for a manual transmission.
Same picture turned right side up. The yellow "X" is the upper hole for 4-bolt cast iron master cylinder.
The top and bottom bolt holes are for the brake booster.
The middle bolt holes are for the 2-bolt aluminum brake master cylinder. There are no shared firewall bolt holes between manual or power brakes (with exception of the firewall stiffener plate).
The '76-77 (or early version) the lower 2 bolt holes are shared with either the 4-bolt (cast iron) or with lower brake booster studs.
I “believe” the power and manual brake master cylinders can be interchange – providing you have the studs and the rubber grommet that holds the pushrod in place.
The rubber grommet (Chrysler calls it a Ring) is a part somewhat hard to find, but I “believe” it is the same as what was used for the late '60's Muscle cars – so it should be available (somewhere). It is also used for power brake cars.
I just purchased these parts from a '79 F-body to install into my '77 wagon (once I make a new pair of holes on firewall).
2-bolt master cylinder firewall stiffener plate. You can see the lower brake booster cylinder holes are in the firewall but are covered by this plate.
Aluminum master cylinder (with push rod and boot in place).
I believe a person can make a new stiffener plate if you have access to drill press, measuring tools, materials and a bit of patience.
Unrelated, these are pictures of a '66-70 B-body manual brake stiffening plate.
A bit different – but mostly the same.
A person could also make their own stiffening late if converting to a 2-bolt aluminum master cylinder or could use a bolt in place bracket (which is another option you have).
he power brakes on my '77 wagon is too strong – so I was considering leaving power brakes in place but replacing the brake booster from a K-car (which is a lot smaller and has less power assist). I have comments and pictures in quite a few posts about using a smaller brake booster (if that was something you are considering).
BudW