Copied from a Chevy forum, found my problem. The brakes may well be bled, but the bore in the master cylinder doesn’t engage with the plunger from the hydroboost. I used a corvette style because the suburban master has 7.5” over center to the cap and corvette is only 3.5”.
Now I’ve gotta buy a compatible master cylinder.
I wanted to put a a little info, hope this helps out.
There are two normal types of master cylinder pistons when it comes to the input seat. Shallow bore (left pict 1) and Deep bore (right pict 1). Most commonly, Shallow bore styles are used with power brakes, and Deep bore types are used in manual set ups. It is now common in the aftermarket to use Deep bore pistons in all master cylinders, and convert them to Shallow bore style by inserting a "plug" into the bore. (shown below the Deep bore master on the right). Here's the catch. The fit from a booster or hydro to the piston should be set at "zero" lash. Think of setting up a solid cam at .000" to .002", basically zero lash. If the output rod from the booster/hydro is not letting the piston return 100%, you may (not always) have some brake drag problems. I have found it to be VERY common, with new aftermarket masters, that the plug is too long. usually .070" to .100", yes, almost 1/8". You should be able to bolt the master flush to the booster/hydro without compressing the piston into the master. I cut the plugs down in a lathe, but they can be ground or sanded.
Another way to adjust this is with the booster output rod (pict 2) most - not all - are adjustable at the tip of the output rod on the booster. Sorry, but most hydroboost unit are not adjustable. Checking the master/booster fit should be standard practice on any new brake install, just like setting valves. Hope that helps out.
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