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.

 

[MrMarty51]

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

I nominate You Super Tech.
You dont miss a thing.

 

[Scottgenex]

Update mostly for fun. Ran some errands in the beast and after 20 mins of city driving the brakes went to crap. So, I got a wild hair and tore apart the PS pump and did a rebuild on it just cause.

Didn't see anything wild that stood out. Very little derbis. Nothing was clogged. Lines were free and clear. Vanes in decent shape. Didn't notice any excessive wear or scoring on anything. Little mesh screen was clean.

Slammed it together and gave it a whirl.
Not much of a change in how things felt. Figured I'd pull the booster apart cause I got nothing to lose at this point.

I know it has a spring in it. Being gentle and slow to not let it launch apart it had some pressure behind it. Pulling the last bolt and trying to hold it together it physically out powered my meat claw and blew apart with a explosive "poof"!

Fluid and parts shot all over like a geyser (safety glasses kids). I left it overnight before taking it off. Guess the accumulator works.

Put everything together and went for a short drive. Steering still feels great as it has. Brakes...ehh. They feel just like when I put the new booster on two weeks.

When I have more time I'll zip it around for a longer time and see if anything has magically healed or not. If I'm a betting man, I'd say no.

 

[Blue Ox]

Update mostly for fun. Ran some errands in the beast and after 20 mins of city driving the brakes went to crap. So, I got a wild hair and tore apart the PS pump and did a rebuild on it just cause.

Didn't see anything wild that stood out. Very little derbis. Nothing was clogged. Lines were free and clear. Vanes in decent shape. Didn't notice any excessive wear or scoring on anything. Little mesh screen was clean.

Slammed it together and gave it a whirl.
Not much of a change in how things felt. Figured I'd pull the booster apart cause I got nothing to lose at this point.

I know it has a spring in it. Being gentle and slow to not let it launch apart it had some pressure behind it. Pulling the last bolt and trying to hold it together it physically out powered my meat claw and blew apart with a explosive "poof"!

Fluid and parts shot all over like a geyser (safety glasses kids). Do they keep residual pressure for a long time? I left it overnight before taking it off. Only reason I can see why is the flush after the rebuilt pump. Wasn't running it dry or low. Either way I assumed the air disapate after a few hours. Could've been me, or some random problem.

Put everything together and went for a short drive. Steering still feels great as it has. Brakes...ehh. They feel just like when I put the new booster on two weeks.

When I have more time I'll zip it around for a longer time and see if anything has magically healed or not. If I'm a betting man, I'd say no.

Hydroboosts have a pressure accumulator which usually needs to be drilled to vent accumulator pressure to make it safe.

I'm glad you didn't get hurt, but next time maybe RTFM. We also have the Internet these days, you know.

 

[Scottgenex]

Thanks, me too.

I thought it might have something to do with not pressing the pedal to relieve the hydraulic pressure as one would do in an accumulator test. I looked around online a bit, read, watched what info was available. Dove in with both feet.

The gas charged chamber has to be drilled/vented for a reseal/gasket replacement on the unit?

 

[Blue Ox]

Thanks, me too.

I thought it might have something to do with not pressing the pedal to relieve the hydraulic pressure as one would do in an accumulator test. I looked around online a bit, read, watched what info was available. Dove in with both feet.

The gas charged chamber has to be drilled/vented for a reseal/gasket replacement on the unit?

I'd have to crack the manual again. It's been a long time. My recollection is the accumulator is there to maintain some braking assist in case of an engine stall or hydraulic assist system failure. So you probably could have dumped the fluid pressure with multiple brake applications. The drilling may have been to remove the accumulator itself.

 

[Scottgenex]

I'd have to crack the manual again. It's been a long time. My recollection is the accumulator is there to maintain some braking assist in case of an engine stall or hydraulic assist system failure. So you probably could have dumped the fluid pressure with multiple brake applications. The drilling may have been to remove the accumulator itself.

That was the impression I got in all my research leading up to this. Chalking it up to a bone head move by me.

 

[Snoots]

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