1975 Plymouth Valiant Manual to Power Disc Brake Conversion

[MickB]

Hi All,

Long time lurker and former A-body owner here (my first car was a '67 Valiant). Now I am helping my 79 year old friend with his 1975 Valiant. He's tasked me with converting his manual front disc car to power assisted, since his legs are getting tired. I'd prefer a bolt-on OE solution, but I am not finding much in the way of parts or information out there.

I know these things are never as simple as they sound, so I need to know:

1. If the interior pedal assembly needs changed (ratio/bellcrank, geometry differences)
2. Does the firewall need to be modified or are the particular reinforcement plates required?
3. Is the power master cylinder different than the manual? Not finding OE power spec. MC anywhere.
4. Is there really a difference between '74-down and '75-'76 years or can I just use the parts listed for the older years.
5. I am seeing mixed reviews about the shiny NEW aftermarket kits that come with booster/MC. They are all also listed for '75 down.

I hope you all can help me with this as I am a bit lost. If I can't figure out more info, I am probably going to hunt down a 75-76 parts car and rob all of the afformentioned parts. That oughta do it...

 

[junkyardhero]

1. If the interior pedal assembly needs changed (ratio/bellcrank, geometry differences)

im pretty sure the pedal can remain the same

2. Does the firewall need to be modified or are the particular reinforcement plates required?

no modification or reinforcement necessary. although, with the rust i'm seeing there you may want to thoroughly inspect the area and assess.

3. Is the power master cylinder different than the manual? Not finding OE power spec. MC anywhere.

the power master is is generally bigger diameter bore. i *think* stock was 1-1/32 or 1-1/8 by then. anything 1" or bigger should be sufficient.

4. Is there really a difference between '74-down and '75-'76 years or can I just use the parts listed for the older years.

the system changed in 73 from a four piston caliper to a single piston caliper (it's more involved than that, but this is keeping it brief). some 74 cars got larger diameter pistons in the calipers. use pretty much anything from 73~76 and you'll be fine.

5. I am seeing mixed reviews about the shiny NEW aftermarket kits that come with booster/MC. They are all also listed for '75 down.

i'm a big proponent of stock stuff.

 

[MickB]

Thanks. After some research, it looks like 1975 is essentially the same as a 1974, or at the very least, a 1974 down booster assy should bolt right in. I have already sourced a NOS Chrysler Power front disc Master Cylinder. I am just looking for the booster and associated brackets, which is proving harder than I thought. And I thought AMC parts were harder to find . And yes, we'll be painting that firewall. The rest of the car is nice.

So the decision is now, should I hold out for an original booster assy or should I just buy one of those shiny Chinesium units they sell? Would it work with the original Mopar 4-bolt style MC?

 

[YY1]

I just looked at aftermarket boosters a few weeks ago.

There are 2 bolt and 4 bolt versions available.

I've heard rumors that the SSBC style 2 bolt units are unreliable.

I did find a 4 bolt version on ebay, that looks like a 1967-ish B body unit.

Was almost ready to buy one and found an OEM 67 B body one over on FBBO.

Just paid for it a moment ago.

 

[MickB]

Nice. I've done a bit of research since yesterday, and have concluded the OE single diaphragm Midland-Ross boosters aren't worth it unless its almost free and already working, since rebuilds are in $400-$500 range nowadays. They also don't offer as much stopping power as the new China-Matic dual-diaphragms. At $139+ tax for new, you can't beat it. I believe it will work with my the OE 4-bolt Power Disc MC I bought. As always, some adjustment/minor fab is probably to be expected.