Thread: Swap to Power Brakes

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My 70 GMC pickup (350/350) has drum brakes and a manual brakes & I want to change to power brakes.

The question is this, is there a different master cylinder for power brakes than there is for manual brakes for the 70 drum/drum combo.

In looking in the LMC catalog, there is only one master cylinder for 67-70.

In 71 it changes to a different one depending if you have manual or power brakes. Is that only because in 71 they had a disc/drum brake combo?


Next is the issue of running the vacuum line to the power brake booster.

I do not have a port at the back of the carb to hook the vacuum line to. I do have a fitting in the manifold that runs to the modulator for the turbo 350.

Would it be ok to put a double line fitting in the manifold and run the trans modulator and the brake booster at the same time? Is this even possible as the vacuum lines are different sizes?

Would it just be best to put in a carb spacer with the brake booster line in it?

Thanks in advance,

Different master and proportioning valve is typically required for a drum swap.

I've never seen a booster share a line with anything, there is probably a reason.

Adding a carb spacer could change engine performance.

Your may need to change wheels to clear the calipers.

You need to run a separate vacuum line for power brakes. You also want to use a vac line directly from the manifold not the carb.

Originally Posted by Bug

Would it be ok to put a double line fitting in the manifold and run the trans modulator and the brake booster at the same time? Is this even possible as the vacuum lines are different sizes?

Any boneyard will be parting some cars and trucks that have vacuum trees on the intake manifolds. Toddle on down to your local yard and you'll find a bunch of 'em. They are used by the factory to provide multiple sources of vacuum from different size lines for different accessories. Just cap off the holes you don't use with black rubber caps from Autozone. Here's a google images link so you know what to look for.......
https://www.google.com/search?hl=en&...39.BQ7bEkiUoh4