1991 C1500 Hydroboost swap issues

Soooo.... I bought some parts and think I got the wrong ones.
I ordered a Cardone 52-7328 (JB8?) off Amazon and it's for a 1991 C2500.
Which that's no problem I can make it fit and mod the Vacuum pedal to line up with the pedal rod.
My issue is I bought a ACDELCO master cylinder that is part number 18M1746.

It doesn't fit the booster. The protruding part of the MC doesn't fit inside the booster. Booster is 1.25" ID I think it was.

Now I'm trying to find what MC I need.

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So i found out the Booster 52-7328 is for 88-91 C2500 and it doesnt come with a mounting plate..

I returned the above part numbers and ordered a 52-7358 Booster and 13-2535 Master cylinder.. Maybe it will work this time.
Going to wait for it to get here and mount it. Then i can see where i need to make a pin on the pedal.

I made a cad model of this pin and you can dl it here for free > https://grabcad.com/library/gmt400-hydroboost-brake-pin-1 . placement of the pin on the pedal rod depends on the length of rod out of the back of the booster, which varies from year/model.

I have converted a few gmt400s to hydroboost and keep in mind the needed pin diameter could be at least 2 different sizes (1/2" or 14mm). All of the hydroboost pedal arms I have seen use the 14mm pin, and the pin is closer to the fulcrum. Here is a picture. Hydroboost pedal on the left, with 14mm pin. Vacuum boost pedal on the right has the 1/2" pin.



If you want to convert your pedal arm to use hydroboost, grind the back of the stock pin and punch it out. Fill the old hole with weld, then drill the new hole closer to the fulcrum, press in the new pin and weld the back.

Also you can use boosters from many different vehicles and the plate will bolt up (like vans, school buses, medium duty trucks etc). With each of these again the rod that comes from the booster will be different length and might require a different diameter pin so keep that in mind.

That is awesome!
Thanks so much for this info.. I already cut my factory pin off. Just waiting on the new booster to get here Tuesday and I'll be able to mock it up and figure out where the new arm lands.
My original pin was 1/2". I already drilled and tapped the rear of it and was gonna reuse it but now you have me wondering what the hole diameter the hydroboost has.
I'll know more Tuesday.

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@gnrl
This new booster (52-7358 Booster) i got has a .627" hole in the arm.
And it looks like it is 7.3125" from the booster mounting plate to the center of the hole where the pin engages it.

I forgot to measure how far off the hole needed to be from the vacuum booster brake pedl to the hydroboost.
It was around 1" closer to the pivot point.
The Hole in the booster arm was .627" so i made a Delrin Bushing and used a 3/8" shoulder bolt. Works perfect.
The bushings on the pedal pivot was worn pretty bad so made a new bushing for that out of delrin also.
Buggered up one of the threads on the hose to the steering gear so that took forever to fix and get it tight.

Gotta get hose clamps and they hydroboost swap will be done!
Thinking about using a drill to prime the system before i put a belt on it.


Also had to make a engagement pin since i couldnt find one. For this booster and master cylinder i come up with 3.375" pin so i made it 3.5" and turned it down on the lathe till i had about 0.20" slack. Only took forever to put together, adjust, and redo.

Ah typical. I didn't mention the 0.627 (16mm) pin because figured they would sending you the correct parts (like you ordered)...but that's the way it goes sometimes.

.627" ~16mm pin is used on later hydroboost vehicles and I have never seen it on a GMT400 truck. I think GMT400s only have 4 possible brake pedals that I know of

15522090 Auto trans vacuum boost (or no boost?) [1/2" pin far fulcrum]
15522091 Auto trans hydroboost [14mm pin close fulcrum]
15522092 Manual trans vacuum boost (or no boost?) [1/2" pin far fulcrum]
15522093 Manual trans hydroboost boost [14mm pin close fulcrum]

Early 2000s express vans have the .627" 16mm pin for sure as I've pulled several. I did the same as you did, made a bushing. Those vans also use the same bolt pattern and actuator rod placement as the GMT400s so they bolt right up which is nice.

RDF1 said:

Also had to make a engagement pin since i couldnt find one. For this booster and master cylinder i come up with 3.375" pin so i made it 3.5" and turned it down on the lathe till i had about 0.20" slack. Only took forever to put together, adjust, and redo.

Click to expand...

Yea the pin between the 2 is another problem. I always get units from junkyard with hydroboost units mated to the master cyl. so I kinda avoid that.

Also if you haven't ran your booster lines yet, I recommend having both the steering return and the booster return have separate ports to the reservior. I've tried twice to Y two 3/8" lines to one 1/2" line and it never works as good for the brakes or steering. You can get away with just putting one cooler on one of the returns and that will be adequate.

I called all the junk yards in Mississippi and couldn't find any with hydroboost and pedal so I went this route.

I do like how the 4 bolt pattern lined up perfect. I've only swapped a gmt800 and had to drill the 4th hole which wasn't so fun.

I used a factory head bolt that had the 10mm head from the 5.3 and put it in a drill to prime the pump. It engaged the front of the power steering pump just right.
Ran it for about 30 seconds at a time for about 3 min and going to try it again tomorrow and see how many bubbles I get.

Bleeding master cylinder and installed the rear wheel cylinders along with rear ss brake line tomorrow so I can bleed the brakes while I have some help...

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gnrl said:

Also if you haven't ran your booster lines yet, I recommend having both the steering return and the booster return have separate ports to the reservior. I've tried twice to Y two 3/8" lines to one 1/2" line and it never works as good for the brakes or steering. You can get away with just putting one cooler on one of the returns and that will be adequate.

Click to expand...

I got to thinking and i dont recall seeing a cooler on this truck. Ill have to find one and plumb it in after i get it running.

gotcha, yea junkyards are hit and miss. Esp now where they only want to pull the exact parts for you. *start rant* I guess people dont care about pulling their own parts anymore but I like walking around the junkyard and finding out stuff that I would never know. *end rant*

Like for instance the ps cooler out of a crown victoria is HUGE, very common (every yard has a crown vic) and you can pick them up for $5, maybe free if you buy other stuff. I never would have known that if I wasnt snooping around the junkyard one day and saw it. My 1990 lt1 1500 has one.

You might not need one either. Every hydroboost gmt400 I've seen had one but it might not have been mandatory equipment.