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Discussion Starter · #1 ·
My 70 GMC has drum brakes and a manual master cylinder. I want to change to power brakes. I can order the parts from a number of sources.

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,
 

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O’Reilly’s Auto Parts confirms the same master cylinder is used for both manual and power brakes on the 1970 GMC. This is not always the case with different GM vehicles. I’ve encountered Chevelle drum brake master cylinders (for example) that have a shallow or deep socket on the back of the master cylinder where the pedal or booster pushrod engages the piston.

If the incorrect master cylinder is mounted on the booster, depressing the brake pedal will act like an on/off switch and the brakes will lock up immediately (dangerously). This is independent of whether the vehicle has disc or drum brakes, it’s a matter of having the compatible booster/master cylinder combination.

So if all you want to do is add a 1970 power booster to your existing drum master cylinder there won’t be a problem.

The vacuum connection shouldn’t be a problem if you have room behind the carb to add a couple of brass fittings. Simply replace the factory fitting with a T, one port suitable for a 3/8” hard line of hose barb to connect to the booster hose, and the other necked down to suit the modulator.
 

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Discussion Starter · #3 ·
Thanks for the info. I have posted this on on a couple of different forums and came up with about 90% with the same response.

Looks like I will pickup a stock booster with brackets, all needed hardware and install.

While I am at it (but after the booster install) I will check all of the brake items at the wheels and see what might be needed.


Thanks again for the info.
 
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