Hydroboost & Master Cylinder - M1008

[ActionStarCBRxx]

I had a bad hydroboost on my M1008. Being a practical kind of guy that I am, I decided to replace the Master Cylinder along with the Hydroboost at the same time. Pulling them off was a cinch... I had two new replacements ready to go and installed them. I added fluids to both the master cylinder and the hydroboost. I then turn the truck on and do the right/left turn to force air out of the hydroboost. So thinking it was pretty easy and I notice I still have no pedal return... or the pedal goes to the floor.

No problem, bleed the brakes right... so since I don't have any pedal to force the brakes to work, I use a vacuum and start pulling nasty stuff out of the lines. Finally all four wheels show clear fluid and I don't see any bubbles. But still no pedal pressure and it goes to the floor.

I recall a friend of mine with a diesel suburban of the same year stating that I need to be sure and pull the rod/spring/clip from the old hydroboost before I turn the old one back in for a core. Hmmm I already returned the old hydroboost so that means I am out a spring/rod/clip. I start on a long search and finally find a dealership in Kansas that can pull all four parts for me (spring/clip/plastic thing/rod). I get the parts two days ago, and pull just the hydroboost and install the rod/spring/clip/plastic. I get everything back hooked up (hyrdroboost/rod/mastercylinder) and I still don't have pedal pressure.

I start the truck up and turn the wheel side to side back and forth to clear the air bubbles and I still don't have any brake pedal... it goes to the floor.

I am wondering if I installed the spring/clip/plastic/rod wrong... or if I need to go back and bleed the brakes again. I looked in one of the manuals and it shows the rod coming out of the hydroboost, but not how it is assembled.

Does anyone have a screenshot of how the rod/spring/clip/plastic cup go in the hydroboost? Is it something else that I am missing?

Thanks!
K

 

[4bogginchevys]

dumb question.....did you bench bleed the master first....I know you bled it but if you got air down in the middle you'll fight it for more than a quart of fluid. No, I dont know how that clip, rod, etc. goes together...good luck.

 

[ActionStarCBRxx]

No I did not bench bleed it... do you think its worth pulling apart and bleeding it that way?

 

[4bogginchevys]

no, not at this point. I would just lay the cover over the master without making a good seal and pump those brakes for all you got. I call it backward bleeding, i'm sure there's a real name for it. It will get fluid all over the place if your not careful, but it will also bleed all the air out from about the proportioning valve to the master. If you had someone helping they could watch for the absence of air bubbles when you've pumped it enough, then bleed it all toward the wheels, it'll save some fluid on the ground. remember rr, lr,rf,lf...good luck

 

[rue_69]

When you say "it goes to the floor" do you mean there is very little resistance and it very easy to push to the floor, or is the pedal literally already completely depressed? Because if there is no resistance in the pedal @ all I would assume you have a bad master cylinder.

 

[ActionStarCBRxx]

I should have stated that there is very little resistance and it very easy to push to the floor. It will return slowlly on its own... but I think thats due to the hydroboost. *shrugs*

 

[Bunkerbuilder]

There is a valve under the Radiator core support that has a button on one end that if I’m not mistaken needs to be pushed or pulled out and clipped.

That is one possible issue, and is the engine running when you are attempting to bleed the brakes?

If not you might be doing it the hard way.

I would also think about purchasing a vacuum bleeder tool to get this job done.

As was said previously bench bleeding would have made this job a lot easier and IMO would be worth pulling the master cylinder off the hydro boost unit to perform this job.

Get 2 pieces of hard line that fit the master and form them into a C or U shape so they end up close to the bottom of the MC.

You will need to cut them to size and clean up the ends so there are no metal fragments to get into the MC.

You may need to buy an adapter for one of the sides as the nut on the end of both tubes aren’t the same size.

If need be Ill post a pic of one of my MC HB spares tomorrow.
LMK

 

[4bogginchevys]

There is a valve under the Radiator core support that has a button on one end that if I’m not mistaken needs to be pushed or pulled out and clipped.

That is one possible issue, and is the engine running when you are attempting to bleed the brakes?

If not you might be doing it the hard way.

I would also think about purchasing a vacuum bleeder tool to get this job done.

As was said previously bench bleeding would have made this job a lot easier and IMO would be worth pulling the master cylinder off the hydro boost unit to perform this job.

Get 2 pieces of hard line that fit the master and form them into a C or U shape so they end up close to the bottom of the MC.

You will need to cut them to size and clean up the ends so there are no metal fragments to get into the MC.

You may need to buy an adapter for one of the sides as the nut on the end of both tubes aren’t the same size.

If need be Ill post a pic of one of my MC HB spares tomorrow.
LMK

Im really curious about this button, what does it do?

 

[ActionStarCBRxx]

Please post a picture of the "U" your talking about... I can't visualize where the line is supposed to go...

shape so they end up close to the bottom of the MC.

I will read the manual tomorrow on bench bleeding as well.

What is the valve your referring to?

There is a valve under the Radiator core support that has a button on one end that if I’m not mistaken needs to be pushed or pulled out and clipped.

Thanks for your help!

K

 

[Bunkerbuilder]

It looks like this.

What is does for the brake system as I understand it is keeps a balance between the two sides of the brake system.

IE the front and rear are two separate systems in that if one loses brake fluid the other side will still work and stop the vehicle.

There is a electrical contact in this valve that grounds the contact when there is a unbalanced condition in the system.

IE when one side loses brake fluid the spool moves to one side or the other and makes contact with the vehicle ground hence completing the circuit and making a light on the dash light up to let you know there is a problem.

Why this button is in there and how it works is something I dont totally understand.

Google can help us both understand this better with a better search than I have done in getting the pic of this valve. .

ETA2: a pic of the tool that holds this valve in when bleeding the brakes.


ETA: the TM page on how the button is pressed.
7-6. SERVICE BRAKE SYSTEM BLEEDING INSTRUCTIONS

Im really curious about this button, what does it do?

 

[Bunkerbuilder]

Ill look for a pic via Google images.

Think of it like this each tube forms a loop circuit for the brake fluid to pump out of the MC outlet and then go back into the reservoir so it does not #1 leak out and #2 so it does not let air back into the brake fluid outlet.

These tubes can be just about any material steel or plastic, rubber etc.

Examples below

Please post a picture of the "U" your talking about... I can't visualize where the line is supposed to go...

I will read the manual tomorrow on bench bleeding as well.

 

[ActionStarCBRxx]

Ohh duh! So obvious now with pictures. You da man! Thanks!

From experience... is it hard to push on the rod to "simulate" the brake pedal? Is there a lot of resistance?

 

[4bogginchevys]

that "button"doesn't have anything to do with pedal pressure, just a warning device. As I said before, back ward bleed it and then bleed it normally. If that doesn't give you real good pedal than they sold you a bad master...

 

[ActionStarCBRxx]

*PROCEDURE*
I finally had a chance today to bleed everything. I left the master cylinder on the truck but removed the brake lines. I then bleed the master cylinder. I connected the brake lines and vacuum bleed all the lines (starting at RR/LR/RF/LF). The brake pedal pressure hadn't changed. It still travels to the floor when you press the pedal.

I then had a helper in the truck, and we manually bleed the lines. This time we did the opposite (LF/RF/LR/RR) and the pedal would build up a little pressure when "pumping the brakes" but it would go back to "no pressure" when you depressed the brake pedal fully.

We did the same thing again, and started in the rear (LR/RR/LF/RF) with the same results.

*NOTES*
When doing the vacuum bleeding I pulled about 1.5 to 2 oz per wheel and I put in the same amount.
When performing the 1st manual bleed, we were still pulling dark brake fluid out, and I was having to fill the master cylinder about 2 to 3 oz per wheel, which was consistent to what it looked like I was pulling out.
When performing the 2nd manual bleed, I did not get much pressure from the RF but the LF was shooting out pretty strong. Finally at each wheel, pulling out 2 to 3oz the fluid was starting to get clear.

I am stuck... The manual says if you dont get pedal pressure, then its a fluid/bleeding issue.

 

[SPECIALTYLC]

BAD MASTER maybe ? Try capping the out ports on the master and see if it makes pressure.

 

[ActionStarCBRxx]

I think thats the best course of action. I will plug the ports and see if I get pedal pressure. If not, then I will replace the "new" master cylinder with the original one and see if that gives me pedal pressure.

*Note to self... don't perform two operations (hyrdo boost and master cylinder)at same time... this 1/2 day replacement is kicking my arse!

Thanks again!

Anyone else have any ideas... please chime in.

K

 

[acmunro]

Where did you purchase your master cylinder and hydroboost ?

 

[ActionStarCBRxx]

Where did you purchase your master cylinder and hydroboost ?

Both were purchased at Oriely's. Both are rebuilt units.

 

[Bunkerbuilder]

How did the on the truck bench bleed go?
Did you have someone watch when you pumped the pedal?
Did the fluid circulate correctly?


And keep in mind when you bleed in the conventional way with 2 people you close the bleeder screw before the pedal hits the floor or you risk sucking air back into the line.

At least that is what I have experienced.

Are you using the correct brake fluid?
IE Silicone dot 5, not 3 or 4 or 5.1 but the silicone fluid?
And did you get the correct MC for use with Dot 5 Silicone fluid?

I bought a few of the MC's when I spotted them on ebay for cheap a few years ago, if you dont have the correct one and want it I might be able to find the source I bought them from, LMK.




ETA: are you aware that there are two rod lengths involving the power unit and the MC ?

IE one is early and the other is late from what I remember.
IE if you have a short rod HB power unit and a deep MC it wont mesh correctly.

Ill go down to the basement tomorrow and take some pics of what I'm talking about if you like.

I would purchase a new correct MC for the truck and take the problem of a rebuilt unit out of the equation.

Both were purchased at Oriely's. Both are rebuilt units.

 

[acmunro]

And keep in mind when you bleed in the conventional way with 2 people you close the bleeder screw before the pedal hits the floor or you risk sucking air back into the line.

At least that is what I have experienced.

Any idea what would cause this ? I have replaced master cylinders on a few trucks civi and military with the hydroboost setup and have had problems getting pedal. Maybe this is it ?