1967 Chrysler Imperial with front disk brakes has a bad power booster. My son wants to use this car in the street class at our local drag track.

What's it take to convert power brakes to manual brakes? Is the master cylinder the same or is there a manual master cylinder that needs to be used instead? Is this even feasible or should he just get the booster rebuilt?

He can remove the booster , the difference in masters is drum brakes vs. disc and reservoir sizing for the front calipers.

Yes drum and disc are different.

He can use the power booster as a manual. He will have to watch his pedal ratio. The manual brake cars have the pushrod mounted higher up on the pedal. This gives more pedal power, at the expense of a greater distance of pedal travel. I'm not sure on a 67 Imperial, but some other mopars have a cantilever that lowers the pushrod from the manual position to the power position. If so, this can be removed and the pushrod mounted direct to correct the ratio.

He's going to have to sort out a pushrod and linkage for the brake pedal to the master cylinder. And get mounting studs for mounting the master to the firewall. There may be a different backing plate for manual vs booster mount for the firewall?
The linkage parts for a manual drum setup would probably work, might need to get an adjustable pushrod, which Dr Diff probably sells. And the rubber retainer to retain the pushrod in the MC piston.
A little backyard engineering involved, but not especially difficult.

manual brakes need a MC with the machined circular groove in the rear piston "thimble" about halfway down for the snip of neoprene retainer on the manual brake pushrod to lock into. PB do not require this "groove" but can use a MC with this groove. all that is left is the bore dia and as mentioned possibly a pedal swap to change the ratio (if needed). if the MC has the groove & it is in good shape I would try it & if the pedal effort and or pedal travel is unacceptable then purchase a different one.

When I worked at Raybestos I noted that most if not all the disc brake mopar master cylinders from the 60s and 70s had the groove for the rubber pushrod retainer.
If the '67 master cylinder doesn't have it, they changed the master cylinder for disc C bodies in '69 to the same style used in B body hemis, that definitely would have the groove as there was a manual disc application of it used on taxis. And it's the same bore diameter 1-1/8".

If I was you I would have the booster fixed
How big is your Son and how strong are his leg muscles? How much does that car (Tank ) weigh
I've owned and driven a bunch of the early large body Mopars including several Imperials, none of them would stop worth a hoot with the motor off when towing them I was a lot younger then and I worked for the local telephone company climbing poles every day so my leg muscle where not weak, I weighed right at 200 lbs. at that time so my legs muscle where strong and stout
Be safe, fix the booster

I totally agree getting the booster fixed. Imperial is a boulevard barge that is not easy to stop. Think of safety first.

Bore size is different

Fix the booster.
The conversion is a lot of parts to come up with and make work.

I can tell ya that the booster failed on my 73 Imperial on the way to a strong 18.0 run at Union Grove. Wi... and I am lucky to be a in one piece... it is a bear to stop without the power working.

Definitely fix/replace the booster, why would you want manual?

And sorry I have to ask, why race a barge like that? Is it for Halloween or something like that? That is the only way I could see taking a car like that to the race track.

He's strong enough to use manual brakes. 5'11" 230

Cab- don't forget vacuum boosted brakes don't push easily without vacuum because of their design. But a manual setup properly should be just fine. It will require more force than a vacuum set up but not nearly as much as a vacuum set up with the engine not running. Haven't weighed it but I imagine this boat is over 4k lbs.

The main reason I'm thinking of converting to manual is for drag racing in the future. If he ever builds any sort of engine with a cam and poor vacuum, then inconsistent brakes would become an issue.

I guess the simplest thing would be to just fix the stock setup that's there now. Won't have to worry about getting the right mounting bracket, pushrod, master cylinder etc. Brake lines to the MC look fine now so they could stay the same if he stays with the stock boosted setup but would have to be redone if the master cylinder was mounted directly to the firewall.

Thanks all!

Since a boat like that didn't have manual brakes as an option you'd be stuck either trying to make something, or make something work.

A properly setup manual brake system isn't any harder to stop than a power setup. Pedal travel will be the only difference.

I've had plenty of power and non-power braked Mopars, both all drum and drum/disc setup and never noticed any difference in how the brakes "felt". Unlike Chevys which were very noticeable to me.