Hydroboost - Hard Pedal

[MrMarty51]

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The front and rear reservoir are swapped size wise, but if they are plumbed right its not a big deal.
The reservoir is bigger than the one on my dads truck

So I suppose bore maybe..?>

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I do not know what the internal differences between the two types is, I have never dismantled either one to find out.
The parts man at that store is a very good friend and had worked as a parts man at one of the local GM dealerships for many years.
With My phone call He investigated what had been sold to that character and what part number was on the box, when the MC in the box got to Him, He checked and while on the ohone with Me declared that the MC that had gone out of the store was for a vacuum assist and the proper MC for that application was in Billings and would be in store the next day.
Thats about all I know about the differences between the two, maybe one digit in the part numbers ?

 

[Scottgenex]

Thanks for those diagrams. For some reason my search didn't pop up anything worth looking at, or they were super blurry and I couldn't zoom in. It does look like I could have the low pressure lines swapped on the respective PS return ports. Not sure that it makes a difference, but hey, it's a freebie fix just to see.

As for the MC, I purchased the replacement based off the vehicle I pulled it off, an 84 diesel truck with JD7 brakes. My rear drums are 13" so all should match up and be happy from my knowledge. It's plumbed as - front larger resvoir for the front discs; rear res for the rear. Verified when I was bleeding the whole system.

 

[AuroraGirl]

Thanks for those diagrams. For some reason my search didn't pop up anything worth looking at, or they were super blurry and I couldn't zoom in. It does look like I could have the low pressure lines swapped on the respective PS return ports. Not sure that it makes a difference, but hey, it's a freebie fix just to see.

As for the MC, I purchased the replacement based off the vehicle I pulled it off, an 84 diesel truck with JD7 brakes. My rear drums are 13" so all should match up and be happy from my knowledge. It's plumbed as - front larger resvoir for the front discs; rear res for the rear. Verified when I was bleeding the whole system.

i noticed your combination valve switch isnt hooked up, maybe your only using your front or rear brakes because its tripped

 

[AuroraGirl]

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[Bennyt]

Just to verify...you did the swap or it was already on and previously working fine?

Do you have the pushrod going to the correct location on the pedal? It is different than a vacuum booster and you sometimes have to drill the pedal if it doesn't have the correct hole.

 

[AuroraGirl]

Just to verify...you did the swap or it was already on and previously working fine?

Do you have the pushrod going to the correct location on the pedal? It is different than a vacuum booster and you sometimes have to drill the pedal if it doesn't have the correct hole.

I didnt even remember that, forgot the pedal had different geometry. Is the whole assembly different or just the pedal arm

 

[Scottgenex]

Combination valve is in place. I installed a new one and currently the block off bleed tool is still inserted rather than the brake light pin thing. I keep forgetting its in. All in all the entire brake system is new.

I did the entire engine swap and restoration over the past few years. Got the hydroboost from the junkyard three years ago. I was unaware the pedals were different. I assumed it was all in the rod length and all that carried over from the hydroboost. Maybe that's a contributing factor. Though two times its happened I've been driving in a straight line on the highway not using brakes at all. Third time was on side streets cause I knew it'd happen and didn't want to have deal with it on I-15.

The brakes have worked fine for two years after the swap. I never really drove the truck very far or long. Just a Sunday cruiser/grocery getter when the weather is nice. No snow driving/wet roads in winter cause of the salt. It wasn't until recently I ventured out further and the problem arose.

When the pedal gets hard theres zero pedal travel what-so-ever. Park for a bit and come back and it's operating normal again. Assuming the air has worked itself out. This only happens when I've run it for 30+ minutes.

I greatly appreciate the advice and help. Don't have too many people around me that I can bounce ideas off of about these vehicles.