Why For?

http://www.fullsizechevy.com/forum/general-discussion/technical-maintenance/494174-stay-tuned-upgrading-GMT400-burbs-brakes-GMT400-hydroboost-GMT800-master-project.html

Did anyone on this forum ever try this? Curious.

The feel I get is that the valve that opens to let the vacuum assist is too small and it takes too long to act so that when jump on the brakes, you have several seconds of 0 assist.

CrazyHoe wrote:

The feel I get is that the valve that opens to let the vacuum assist is too small and it takes too long to act so that when jump on the brakes, you have several seconds of 0 assist.

That is a one way valve, and should draw enough vacuum under high vac operation to operate the brakes 2-3 times with no vacuum. If you are getting this feel it sounds to me like the vacuum valve on yours might be blown out, or you have a leaky membrane inside the booster.

I just did this mod on a 99 Tahoe last night... a WORLD of difference. This is DEFINITELY the way to go. I'll post some more details when I get a chance to sit down and do so.

Soft GMT400 brake pedal GONE.

This could very well be the best GMT400 mod around, especially in terms of bang for the buck. I feel a lot better with my mom driving this Tahoe around now that the pedal doesn't practically go to the floor. Even with new brakes at all 4 corners, a thorough flush of the system (including ABS modulator), SS braided hoses, and rears adjusted out, the brakes were NOTHING like this before. It's due for another brake job now, but I wanted to see what difference the master cylinder alone would have with the old, worn pads & shoes. It will only get better from here, as I'm about to do all new brakes at all 4 corners.

Very, very pleased.

Speeder wrote:

CrazyHoe wrote:

The feel I get is that the valve that opens to let the vacuum assist is too small and it takes too long to act so that when jump on the brakes, you have several seconds of 0 assist.

That is a one way valve, and should draw enough vacuum under high vac operation to operate the brakes 2-3 times with no vacuum. If you are getting this feel it sounds to me like the vacuum valve on yours might be blown out, or you have a leaky membrane inside the booster.

Vacuum still present after 2days parked.

JR96CK wrote:

I just did this mod on a 99 Tahoe last night... a WORLD of difference. This is DEFINITELY the way to go. I'll post some more details when I get a chance to sit down and do so.

Soft GMT400 brake pedal GONE.

This could very well be the best GMT400 mod around, especially in terms of bang for the buck. I feel a lot better with my mom driving this Tahoe around now that the pedal doesn't practically go to the floor. Even with new brakes at all 4 corners, a thorough flush of the system (including ABS modulator), SS braided hoses, and rears adjusted out, the brakes were NOTHING like this before. It's due for another brake job now, but I wanted to see what difference the master cylinder alone would have with the old, worn pads & shoes. It will only get better from here, as I'm about to do all new brakes at all 4 corners.

Very, very pleased.

That's awesome! I've got 2 weeks off so as soon as you post the details I'll be going this way too.

Here's a quick rundown:

I used a master cylinder from a 2002 Tahoe/Pickup application. I went with a new AC-Delco unit from RockAuto. They show 2 different ones: part numbers 18M1159 (Durastop) and 174-1003 (OE Service). I contacted AC-Delco and they told me they are both EXACTLy the same unit, just sold under the two different names (Durastop and OE Service). The Durastop brand is simply a moniker beacuse GM will not allow them to sell this part to fit other makes and models under the original GM number. They both carry the same 12-month 12,000 miles warranty through AC-Delco. I used the 18M1159 unit.

As far as bolting up, there are no problems there, but you will need to adapt the GMT400 forward port hard line to the GMT800 master cylinder forward port. The GMT400 unit uses a 1/2"-20 thread on the rear port and a 9/16"-18 on the front port (note: this does NOT refer to front or rear brake fluid circuits but literally to the physical locations of the port front to rear on the side of the master cylinder). The GMT800 unit is 1/2"-20 thread on both froward and rearward ports. The rearward port obviously bolts right up, but you will need to adapt the front port to the existing GMT400 hard line. For this I used an Edelman #258306 adapter fitting, as it is a Zinc-plated steel unit. There are others available such as the AGS #BLF-26C (which is brass) or the Weatherhead #7912 (brass as well, I believe). Again, I chose the Edelman unit because it's Zinc-plated steel. I got it from Summit.

If you want, you could forgo the adapter and just re-flare the hard line and use a "correct" fitting. This would look a bit more factory, and I may go back and do this later. For this, you will need an Edelman #121045 fitting, which is the correct 1/2"-20 thread but will slide right onto the larger 1/4" hard line. Again, I got these through Summit. They are an olive-colored finish, which is what my factory pieces are on my 96 pickup. However, this 99 Tahoe had grayish-colored factory fittings, so in this instance I would opt for the AGS #BLF-44C-5 tube nuts, which look identical to the factory gray pieces (yes, I'm extremely picky about this kind of thing). The appropriate 45-degree double flaring tools are required to make a new double inverted flare on the hard line once you have cut off the old flare and swapped out the tube nuts. The easier way is to use the aforementioned adapter in the above paragraph, but this is the way to do it if you dont want to have any extraneous joints in the hard line.

A bench bleed of the new master cylinder is of course required before you bolt any of this up, but I assume everyone here knows that and how to go about doing that.

Once it's all bolted up and ready to bleed, bleed all four corners (right rear, left rear, right front, then left front). You will then need to do the "Automatic Bleed Procedure" on a suitable scan tool to purge the ABS modulator. The service manual states this is mandatory when replacing the master cylinder. However, looking at a cross section of the ABS unit (Kelsey-Hayes EBC-310 model), it appears that one does not necessarily introduce air into the dump valve area by replacing the master cylinder, but I did not opt to dig any further into the issue or experiment here. I employed my old Snap-On "Brick" scanner (MT2500) to perform the Automatic Bleed Procedure. You will need the later model "black" programmable cartridges for this, as the older non-programmable cartridges do not have this procedure on the ABS menu (at least mine did not... I had to upgrade to the black cartridges). The scanner will guide you through the procedure... basically, just keep your foot pressed down on the pedal as it cycles the pedal up and down a few times via the ABS pump. At this point, bleed all four corners again in the aforementioned order and you're done.

Forgot to mention this earlier: the GMT800 master cylinder has a low fluid level sensor on the side. I've not done so yet, but I plan to tie this into the brake warning light on the instrumnet cluster. I looked this up a while back using the intended 2002 model year truck as a guide, and the required connector is a #12162194 Delphi 2-way Metri-Pack 150.2 Series Connector in black (Mouser lists it as gray, however. I believe the mating connector is black). The wires are purple and black, circuits 333 and 550 respectively. I'm sure EFI Connection has suitable wire to re-create that. If you want to go about it the easier way, AC-Delco pigtail replacement is #PT1789. I plan to get the pieces and re-create the 2002 factory wiring here and simply tie into the parking brake switch circuit to activate the warning light.



Last edited by JR96CK on Fri Jul 20, 2012 4:42 pm; edited 3 times in total

I just gt done adding the hydro boost to my truck. Big difference. When it comes time to replace pads, I will upgrade to stainless steel lines and the 3/4 ton calipers.

What is the difference between the Mastercylinders of the GMT 400 and 800 trucks. I just found out not realizing the difference in size between the vaccum booster master cylinders to the hydro boost. It is too bad a person could not go to the bigger drums and shoes if you didn't go to disc brakes on the rear with out having to go to the 8 lug axle. This is on the 9 1/2" rear end. It is just interesting to learn about all the things you can do to improve on your trucks performance all the way around.



Last edited by Chevy 97 on Fri Jul 20, 2012 7:29 pm; edited 1 time in total

JR I was looking at doing this as welll but could not find answers to a few questions to clarify:

Did you do this on a vacuum set up or a hydroboost setup?

I was at the parts store and compared MC's for '98 and '02 a while back. Did you have to make any modifications to either the MC or booster? I ask because by the naked eye the '02 MC piston/valve appeared to be a tad larger.

Can you confirm that the brake code for your mom's Tahoe is JB6?

I am all set to install a JB6 rear end from a tahoe with JB6 brakes on my truck. However I need to confirm that the two housings are interchangeable.

Thanks,
TJ

The two master cylinders side-by-side:












GMT400 piston bore diameter (Inch):



GMT400 piston bore diameter (Metric):



GMT800 piston bore diameter (Inch):



GMT400 piston bore diameter (Metric):



GMT400 housing diameter (Inch):



GMT400 housing diameter (Metric):



GMT800 housing diameter (Inch):



GMT800 housing diameter (Metric):



Booster inner diameter (Inch):



Booster inner diameter (Metric):



GMT400 housing depth (Inch):



GMT400 housing depth (Metric):



GMT800 housing depth (Inch):



GMT800 housing depth (Metric):



It should be noted in the pics below which show the available mounting depth in the vacuum booster that at the base of the engagement pin is a black rubber gasket which allows for quite a lot of compression against it. Therefore, a master cylinder with a bit more depth in this area will fit fine, it just compresses that rubber gasket a bit more. Not an issue.

Booster depth (Inch):



Booster depth (Metric):



Mounted up and connected:






Close-up of the fittings and adapter:







So now the brakes are awesome, I like the greater volume in the reservoir (much nicer when you're bleeding it, as you don't have to top off as often), and the thread-on cap is far superior to that stupid gasketed snap on style cap GM insisted on using from the early 80s through the GMT400 years. I hated those caps.



Last edited by JR96CK on Fri Jul 20, 2012 8:19 pm; edited 1 time in total

98Blackss wrote:

JR I was looking at doing this as welll but could not find answers to a few questions to clarify:

Did you do this on a vacuum set up or a hydroboost setup?

I was at the parts store and compared MC's for '98 and '02 a while back. Did you have to make any modifications to either the MC or booster? I ask because by the naked eye the '02 MC piston/valve appeared to be a tad larger.

Can you confirm that the brake code for your mom's Tahoe is JB6?

I am all set to install a JB6 rear end from a tahoe with JB6 brakes on my truck. However I need to confirm that the two housings are interchangeable.

Thanks,
TJ

TJ,

These are JB5 brakes with vacuum boost. 99 Tahoe 4wd. Bone stock braking system except for the braided stainless steel lines I put on it a few years back. Absolutely no modifications were required aside from using that one adapter fitting to get the threads to match up on the forward hard line (as pictured).

Thanks for the quick reply! This great news that this works on the vacuum booster setup. There is a lot on this with the hydroboost. I will be doing the front pads tomorrow so I too will try to get this done as well. I will see how much luck I have finding this adapter here in Denver.

TJ

That Is interesting on the master cylinder depth. Is the flange to piston dimension deeper as well? I had heard somewhere that the reason the vaccum brakes had a spongy/long travel pedal was becuse the booster pushrod was to short and there is actually a gap between the pushrod of the booster and the master cylinder piston.

I need to do something to help the brakes on my dads 96, He has hand controls on it and makes for a totally unsafe ride for him that way but a H-boost swap is out of the question this would be a simple cheap upgrade for him.

Does anyone know if the adapter fitting is available through someone like autozone?

My local autozone has the AGS #BLF-26C in stock on the shelf.

TJ

O'Reilly carries Edelman fittings. Not sure if they stock that adapter, but worth a shot. I just opted for Summit because it was a bit cheaper and shipping is always fast with them. Have always had good service from Summit, actually.

(EDIT: Just checked and some of my local O'Reilly stores do have it in stock. Looks like you have several stores around Salt Lake, so I would imagine at least one would have it.)

As far as the piston to base depth measurement, that's one I didn't think to look at. Wish I had now. When I install the next one on my 96 pickup, I'll be sure to take a look. Of course, I still have the original master cylindre, so I can provide measurements off of that one. If someone gets hold of a GMT800 unit to do this swap and can measure theirs, then we will know.

Now for the rear JB5....

I believe my rear brake cylinders are 1" in diameter, there is a 1 3/16 model for an increase of 41% in hydraulic pressure. Just a suggestion for when you'll re-do the rears...

Was thinking about that, as I just found out about those larger wheel cylinders and also the feasibility of swapping in one-ton calipers onto a 1500 (not sure if 2wd or 4wd makes a difference here). Unfortunately, I just put new wheel cylinders on the 99 Tahoe, so I won't be swapping those out any time soon. I suppose I may do some experimenting on my 96 pickup in the future.

Ok but the 2002 Tahoe/Pickup uses disc brakes in the rear right?

So with the different diameters and pressures are you able to lock the rear?

Or does the different master cylinder not matter and it's solely the job of the proportioning valve to control the rear pressure and volume?