Bolt size confusion and stuck master cylinder

[R8rPhan]

So, I've been driving around with a leaky master cylinder for 2 years since I bought and resurrected this old square body...

I grabbed a power brake booster and master cylinder off a junked 88 suburban/tahoe about the same time I got this truck and it's been sitting in a bucket all this time out on the front porch...

Today, since I've been snowed in for a couple weeks and can't go anywhere anyway and finally got a relatively dry day, I decided to do the conversion to power brakes.. The bolt pattern on the cab was for power brakes but someone had jury rigged in a master cylinder from something else..

Other than having to drill out the brake pedal pin hole on the end of the master cylinder push rod, the thing bolted right in...

So two issues..

First of all, I didn't have all the nuts for mounting the thing.. The two nuts that were on the existing master cylinder required a 15mm socket, and they were threaded on two bolts that were stuck from the inside, through the pedal bracket, the firewall and the master cylinder flange, with a nut on them on the engine side..

So, I put a 3/8" bolt and nut to replace one of those, used the other one where I found it, and took the remaining nut and threaded it on one of the four remaining posts..

So I need more nuts... But what size are they.. According to every bolt chart I find on line, there is no bolt that requires a 15mm socket.. But require it they do..

Anyone know what size nuts I need for the master cylinder mounting studs? I tried threading a 3/8" nut on them, and they started (less than a turn) but that's it.. So I'm thinking they are definitely metric sized studs...

Second issue, after mounting the booster and master cylinder, I filled the cups with brake fluid and started pumping the brakes to flush the thing out.. But only the rear ports spewed fluid.. The front port/reservoir.. nada..

So I took a paper clip and shoved it in the outlet from the master cylinder and as far into the drain inside the front reservoir as the paper clip could reach and pumped some more.. nada.. Did this several times and nada.. however, I noticed the last time, some tiny bubbles came up inside the reservoir..

I suspect that there's a check valve in there that is stuck from sitting dry for two years.. Any advice on getting the thing to free up and start pumping? I'm hoping that just sitting with fluid in it for a couple days while I finish the other stuff I plan to do will lubricate and free up everything inside, but if it doesn't is there anything else I can do?

Rear brakes pump up and hold pressure and are ready to bleed..

I was thinking of trying to blow compressed air back in through the outlet port, but I don't know what size fitting I need to get to pull that off...

The master cylinder 'should' be good, but sitting for two years in a moist environment probably wasn't the greatest thing for it... Still have to run vacuum to the drum, and have a new check valve for that, but I don't expect issues there...

If I have to buy a new master cylinder, are they all pretty much interchangeable for that year? The junker is long gone so I can't get any more info on it than it was an 88 burb/tahoe...

Ideas?

 

[bucket]

15mm nuts would be for a 10x1.5 mm stud. That's usually how it works with GM products.

Equivalent bolts or nuts from the hardware store would generally take a 16mm or 17mm socket.

 

[Rusty Nail]

That story ought to rated "R". How terrifying. The absolute last thing one should cheap out on is brakes. That's not even my opinion...all of my ideas regarding the tale of horror you tell so well - involve pulling out $40 and buying a new, re-man, whatever! master cylinder that is correct for your 1973 application.

That is your responsibility not only to the truck, but to you, your family, and every other driver on the road, which includes ME, my family, and all of US.

 

[R8rPhan]

That story ought to rated "R". How terrifying. The absolute last thing one should cheap out on is brakes. That's not even my opinion...all of my ideas regarding the tale of horror you tell so well - involve pilling out $40 and buying a new, re-man, whatever! master cylinder that is correct for your 1973 application.

That is your responsibility not only to the truck, but to you, your family, and every other driver on the road, which includes ME, my family, and all of US.

I'm doing the best i can right now.. That's all I can do..

 

[R8rPhan]

15mm nuts would be for a 10x1.5 mm stud. That's usually how it works with GM products.

Equivalent bolts or nuts from the hardware store would generally take a 16mm or 17mm socket.

Thanks man.. 10mm is what I was guessing, but wasn't sure if there was a 9mm or something... But it's not like I can take the truck with me to try them out in the parking lot... Weird that none of the bolt charts even list 15mm head size.. but yet they make the socket and wrenches... They must be for 'something'

Gonna try and catch a ride into the local mountain town (about 10 miles) tomorrow or firday morn to see if they have the nuts at the hardware/grocery store..

 

[Blue Ox]

You could take one of the nuts with you, find a bolt that it threads on to, then you'll know the thread size.

You should probably try bench bleeding the master cylinder before you install it. Not hard to do and it solves a lot of air problems that are extremely difficult on the vehicle. I'm sure there are instructions here that a search will find.

And I have to agree with Rusty. There are way too many changes in the braking systems over the years and even within the years to take a chance on a random master being the correct one.

 

[75gmck25]

Your truck also has a proportioning valve that is designed to shift all pressure to front or back brakes if there is a brake failure. When pressure is uneven (front to back) it moves the pivot inside the valve and it will usually stick in that position and only provide pressure for either front or back. This video provides a fairly good explanation of how it works and how to reset it.

xc_hide_links_from_guests_guests_error_hide_media

The proportioning valve is on the drivers side of the truck, mounted up under the area near the cooling fan shroud. It will have all the brake lines connected, and also an electrical connection for the brake warning light.

Bruce

 

[R8rPhan]

You could take one of the nuts with you, find a bolt that it threads on to, then you'll know the thread size.

You should probably try bench bleeding the master cylinder before you install it. Not hard to do and it solves a lot of air problems that are extremely difficult on the vehicle. I'm sure there are instructions here that a search will find.

And I have to agree with Rusty. There are way too many changes in the braking systems over the years and even within the years to take a chance on a random master being the correct one.

That's essentially what I did, only on the truck.. I don't have a vise, so using the truck as one is my only way...

So which one do I get? I know the cab isn't a 73.. I 'think' it might be from a 79 1 ton farm truck.. Bed, chassis, engine, cab, transmission, all from different vehicles..

I know this power brake drum and bracket fits better then the drumless one that was on it..

Trying to avoid taking any of the nuts back off ... They were a royal PITA to install.. Between the bracket and the drum, a short socket was too short to adequately reach things inside the bracket and a deep socket couldn't fit in there.. Plus there was extremely reduced range of motion for the ratchet.. I probably spent an hour and a half just getting the three of them on there are now..

Was hoping someone knew what was typical so I could avoid taking one off again..

 

[R8rPhan]

Your truck also has a proportioning valve that is designed to shift all pressure to front or back brakes if there is a brake failure. When pressure is uneven (front to back) it moves the pivot inside the valve and it will usually stick in that position and only provide pressure for either front or back. This video provides a fairly good explanation of how it works and how to reset it.

xc_hide_links_from_guests_guests_error_hide_media

The proportioning valve is on the drivers side of the truck, mounted up under the area near the cooling fan shroud. It will have all the brake lines connected, and also an electrical connection for the brake warning light.

Bruce

It wasn't pushing fluid out of the front outlet 'without the lines connected' (ie bench bleeding style), so the issue can't be the proportioning valve.. It wasn't involved at all...

Interesting video though.. Learned a few things..

Anyone know where to get that little part for bleeding the brakes he's talking about at the very end (or at least what it's called)?

 

[Frankenchevy]

@R8rPhan , pm sent. I pulled an entire working brake system off my k30 to replace with new stuff. You’re welcome to the master cylinder, nuts, bolts, etc.

Not sure about compatibility. Parts are from a k30 hydroboost.

(Disclaimer)
I make no warranties as to the working condition of the parts. You’ll have to verify the condition and safety of the parts yourself.

 

[Blue Ox]

That's essentially what I did, only on the truck.. I don't have a vise, so using the truck as one is my only way...

So which one do I get? I know the cab isn't a 73.. I 'think' it might be from a 79 1 ton farm truck.. Bed, chassis, engine, cab, transmission, all from different vehicles..

I know this power brake drum and bracket fits better then the drumless one that was on it..

Trying to avoid taking any of the nuts back off ... They were a royal PITA to install.. Between the bracket and the drum, a short socket was too short to adequately reach things inside the bracket and a deep socket couldn't fit in there.. Plus there was extremely reduced range of motion for the ratchet.. I probably spent an hour and a half just getting the three of them on there are now..

Was hoping someone knew what was typical so I could avoid taking one off again..

Do you own a C-clamp? There must be some vertical object like a garage stud or sawhorse that you could use to clamp the MC and try to bench bleed it. Do you have a combination wrench? I'm sure it's tedious, but it can't be that hard to bolt up a vacuum booster.

bucket is probably right about the stud size, but all of a sudden you're changing the story. Your profile says you have a 1973 truck, and I promise you there was no metric hardware on a 1973. So now the story changes to a 79, which probably still isn't metric since GM only went metric in 1980 and then we get a hodgepodge truck that came from twenty different vehicles. There's probably some people here that can help you, but not without proper information. Especially when you ask what was "typical" for Frankenstein's monster.

I'm not trying to be a d-bag about this even though I come across that way, but you're asking for a golden bullet to kill this werewolf in your driveway, but that magic solution doesn't exist. We'll need a LOT more engineering information to try to find the right pieces to keep you from killing yourself and the people on the road around you.

 

[R8rPhan]

Do you own a C-clamp? There must be some vertical object like a garage stud or sawhorse that you could use to clamp the MC and try to bench bleed it. Do you have a combination wrench? I'm sure it's tedious, but it can't be that hard to bolt up a vacuum booster.

Ha! I have a combination wrench, a flex combination ratchet end wrench, etc.. It's was a serious SOB.. and the fact that the middle two are 'bolts' that have to be passed through the brake pedal bracket on the inside of the cab out through the booster mounting bracket makes it that much worse... I had to use vice grips on the inside to hold the bolt so I could tighten the nut on the outside, and that was fun trying to start the nuts without it all falling back out on the inside, not to mention just getting a vice grip on the things in the first place.. Lots of bad words there...

The problem is that the nuts sit down inside of a channel created by the mounting bracket, and the booster is too close to allow the use of a deep socket.. Then there all the crap in the way preventing motion of the tool...

bucket is probably right about the stud size, but all of a sudden you're changing the story. Your profile says you have a 1973 truck, and I promise you there was no metric hardware on a 1973. So now the story changes to a 79, which probably still isn't metric since GM only went metric in 1980 and then we get a hodgepodge truck that came from twenty different vehicles. There's probably some people here that can help you, but not without proper information. Especially when you ask what was "typical" for Frankenstein's monster.

I'm not changing any story.. The truck is registered as a 73.. It is in fact many different years (as I stated).. What the heck am I supposed to call it?

It's metric... SAE doesn't fit... In fact, all the screws in the dash and heater system are 7mm too.. and I know I am the first person to ever put a socket on some of them... But they're 'all' 7mm throughout the dash (glove box hinges, stuff holding the gauge cluster together, the dash cover, etc.)..

 

[R8rPhan]

I've been doing some reading since last night.. and this master cylinder will cause me problems even if I get it working... It's made for a disc/disc vehicle and I have a disc/drum vehicle... (residual valves, piston travel required, etc.) .... Just not gonna work..

So, I'm gonna scrape together whatever cash I have to and talk someone into giving me a ride to go get a reman master cylinder for a 79 pickup... I'm just gonna 'hope' that it all bolts up okay to the booster... If not, then that's gonna be hard to come up with the money for right now and things are gonna get dicey...

I have some work to do, but can't do it without the truck, and can't fix the truck till I make some money... The weather this winter has been a job killer (relentless)! Even with the truck, there's no guarantee I can get into the properties where the work is..

Been snowed in for 2 weeks now, half of that without power... Had constant storms for the last 6 weeks...

 

[R8rPhan]

@R8rPhan , pm sent. I pulled an entire working brake system off my k30 to replace with new stuff. You’re welcome to the master cylinder, nuts, bolts, etc.

Not sure about compatibility. Parts are from a k30 hydroboost.

(Disclaimer)
I make no warranties as to the working condition of the parts. You’ll have to verify the condition and safety of the parts yourself.

Shipping from wherever you are is probably as much as a reman master cylinder if not more... and no UPS/FedEx truck is even gonna 'try' to get in here right now anyway..

Thanks for the offer.. I'll keep it in mind as plan B if plan A fails...

I'm gonna try to get a reman master cylinder and hope it all works okay... I'll take this one from the 88 down with me and make sure I get something that requires the same length push rod and is for a disc/drum truck... If I can't find what I need then I'm gonna have to figure out how to get some moola together for a booster too...

 

[Blue Ox]

Ha! I have a combination wrench, a flex combination ratchet end wrench, etc.. It's was a serious SOB.. and the fact that the middle two are 'bolts' that have to be passed through the brake pedal bracket on the inside of the cab out through the booster mounting bracket makes it that much worse... I had to use vice grips on the inside to hold the bolt so I could tighten the nut on the outside, and that was fun trying to start the nuts without it all falling back out on the inside, not to mention just getting a vice grip on the things in the first place.. Lots of bad words there...

The problem is that the nuts sit down inside of a channel created by the mounting bracket, and the booster is too close to allow the use of a deep socket.. Then there all the crap in the way preventing motion of the tool...

Alright, cool, I can see that it's no fun. And a lot of the hardware is normally brazed on so it's just a simple stud and not necessarily a two man operation. You have to deal with what you're dealing with. It's all good.

I'm not changing any story.. The truck is registered as a 73.. It is in fact many different years (as I stated).. What the heck am I supposed to call it?

It's metric... SAE doesn't fit... In fact, all the screws in the dash and heater system are 7mm too.. and I know I am the first person to ever put a socket on some of them... But they're 'all' 7mm throughout the dash (glove box hinges, stuff holding the gauge cluster together, the dash cover, etc.)..

You didn't state that in your initial post and your profile says you have a '73. That's all we have to go on. So before you go and buy a master, are we confident we know what braking system we are dealing with? Again, if it's a hodgepodge we can't help you. If you're sure it ALL came from a '79 K30 there's probably a spec we can use to pick out a serviceable master, but a K30 should have had hydroboost, I got the impression you had a vacuum booster.

Different cabs are just tin and it's not going to cause more than heartburn. Brakes, OTOH carry a lot of liability so you want to be sure you're relying on the guys with engineering degrees. From your other post, it looks like you're trying to do your homework, we just don't want to give you uninformed advice that leads to problems.