Long, long brake pedal.

[Pinger]

Looks correct to me.

Regarding what I mentioned earlier, pull that one up side by side with the ACDelco Professional 18M712.

https://www.rockauto.com/en/moreinfo.php?pk=113321&cc=1353057

They appear identical to the point that it looks like they're both using the exact same photos, just in different order and at different resolutions. Take note of the little stuff like the lettering on the cap and the little upside down "SC" stamped into the flange. You can see other examples of this if you look at Raybestos R-Line brake drums for your truck vs. the comparable ACDelco Pros. I think it's safe to assume that Raybestos supplies many of the ACDelco Professional brake parts.

There's slight differences in the casting finishes in the photos but thanks for confirming the Raybestos part as suitable for my truck. I'd not noticed before but if you hover over the part number there's a pop-up list of vehicles it suits. Trusting that RA have this correct as the part number on my MC (on its underside - read with a mirror) doesn't match any on any of the MCs on the RA page.....
It's ordered and should be here on the 9th - just in time for work commitments.

Thanks for the help everyone - much appreciated.

 

[RichLo]

Since your replacing it, why not upgrade to a GMT800 master or hydroboost setup?

 

[Pinger]

Since your replacing it, why not upgrade to a GMT800 master or hydroboost setup?

I have the HydraBoost already and recent threads suggest changing to the GMT800 MC isn't an upgrade but the wrong part.
And the small matter of having to get this sorted and back on the road ASAP for work commitments.

 

[df2x4]

There's slight differences in the casting finishes in the photos

I believe you, but just curious if you could provide any examples specifically? To my aging eyes on my crappy 42" 1080P TV the photos look identical other than size.

Trusting that RA have this correct as the part number on my MC (on its underside - read with a mirror) doesn't match any on any of the MCs on the RA page.....

Does it match any of the "Alternate/OEM Part Numbers" down at the bottom of the page on those listings? Specifically I'm curious about 18029968. From what I can tell according to Nemiga Parts that should be the OEM GM master cylinder part number for your truck. Discontinued, unfortunately.

 

[east302]

They appear identical to the point that it looks like they're both using the exact same photos, just in different order and at different resolutions. .

They do that a lot, I’d agree that Raybestos and Delco Professional look to be sourcing from the same supplier.

Kind of makes you wonder “who” actually makes parts these days.


Sent from my iPad using Tapatalk

 

[Pinger]

I believe you, but just curious if you could provide any examples specifically? To my aging eyes on my crappy 42" 1080P TV the photos look identical other than size.

Mea Culpa! (Latin for 'I fuct up' ).
For sure there was one that had a dimple machined in its blind end and a small amount of machining somewhere else on the same casting and looked slightly less sand cast - more finished. And I can't find it on any of the options now. It was one of the options where there were plenty photos and I'd swear it was the ACDelco one but when I look now I agree fully, the two items look identical and probably are the same photos.
In my defence! - Rock Auto change their listings on whim. Eg, Thursday night, earliest ETA for the Raybestos MC showed as 10th Dec. When I came to order it on Friday that had changed to the 9th.

Does it match any of the "Alternate/OEM Part Numbers" down at the bottom of the page on those listings? Specifically I'm curious about 18029968. From what I can tell according to Nemiga Parts that should be the OEM GM master cylinder part number for your truck. Discontinued, unfortunately.

Part number on mine is 18012985 (assuming I've read it correctly with a mirror) and '389 75 80' is also there but stamped - not cast. Couldn't find that number on any of RA's listings but when I hovered over the 'MC390257' Raybestos part number a pop-up had a list of vehicles it fitted and mine was included. That, and that I'd accessed that page by entering all the vehicle details kind of throws it back to RA if it's wrong. I just had to get it ordered as needed ASAP.
At a guess, if it is wrong it will be internal dimensions. Presumably I will have a MC I can fit and a usable truck again until RA correct their mistake.

Kind of regret not changing the MC earlier - I got blindsided by someone suggesting that the PAS pump could be a cause. One of those things that had to run a little longer to fail a bit more and offer up a more certain diagnosis. The braking fading in the mid part of the pedal travel leaves me in little doubt that the MC is done. And also points to the danger of bleeding by the pedal stroke method. Definitely looking to do it differently in future. To that end, I want a good look at the bottom of the reservoir where the fluid leaves to see what can be coupled there to pressurise without having to pressurise the reservoir body. Going by what I've read, there's a fitting for that purpose.

 

[df2x4]

At a guess, if it is wrong it will be internal dimensions. Presumably I will have a MC I can fit and a usable truck again until RA correct their mistake.

I'm 99.9% sure it'll be fine. I looked up your truck on the Raybestos website and they only list two master cylinder part numbers, one for vacuum boosters and the one you got for hydraulic (hydroboost).

To that end, I want a good look at the bottom of the reservoir where the fluid leaves to see what can be coupled there to pressurise without having to pressurise the reservoir body. Going by what I've read, there's a fitting for that purpose.

I believe you're correct. I've seen it mentioned in the factory service manuals and in this video (although he doesn't use it).

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He references that page in the '98 factory service manual at the six minute mark.

 

[Pinger]

I'm 99.9% sure it'll be fine. I looked up your truck on the Raybestos website and they only list two master cylinder part numbers, one for vacuum boosters and the one you got for hydraulic (hydroboost).

My usual paranoia about ordering the correct part. Compounded by the 10 day lead times and in this particular instance the need to get back on the road ASAP for work commitments. Part should be here on Tuesday - so I can scrape myself off of the ceiling now.

I believe you're correct. I've seen it mentioned in the factory service manuals and in this video (although he doesn't use it).

Shurkey (I think it is) is a fan....
At a guess it is older MCs with a bit of corrosion further down their travel that are susceptible to this type of failure. You (or is that me?) start of with the best of intentions and try to avoid going full travel but there's some other problem and next thing the pedal is on the floor and the MC shot. My 'ol man used to tell me 'the road to Hell is paved with good intentions'.

xc_hide_links_from_guests_guests_error_hide_media

He references that page in the '98 factory service manual at the six minute mark.

How are you getting on with your Tech II? You mentioned it is a clone - cheaper than the 'official' ones? Link to where you bought it from???

 

[df2x4]

How are you getting on with your Tech II? You mentioned it is a clone - cheaper than the 'official' ones? Link to where you bought it from???

Haven't had a chance to do much with it other than pull DTCs, but it does a great job of that! It'll show every single fault even if said fault hasn't been triggered on enough consecutive drive cycles to trigger a check engine light. So I get to be nervous about all kinds of extra stuff. I've been meaning to look at some things in the data stream but haven't had the chance to play with that yet.

It was around $350 US, I bought it from the seller "ronvias" on eBay.

Word of warning though, I bought it almost completely based on the fact that it came with the plastic storage case. It came with a plastic storage case, but one of significantly lower quality than what was pictured. You can get them from other sellers for $250-$300 without any type of case. Which is what I would have done if I'd known better.

 

[Pinger]

New MC arrived yesterday and is currently being bench bled.
There is a slight difference in the spacing between the outlet ports. The forward one is in the same place on both but the rearward one is about 5-10mm out. Expecting the pipe to accommodate this....

Bench bleeding is a bit odd. I do the pump stuff, no more bubbles. Come back again after a few minutes, more pump more bubbles. Rinse repeat rinse repeat....

No defined 'fitting' for bleeding that would allow the reservoir to go unpressurised. Just a well in the reservoir moulding about 3/4'' ID and about the same in depth. Maybe a snug fitting bit of hose attached to a pressure source of choice might work but I haven't the time just now to try right now. If the MC that is about to be removed is the same I can experiment later.