Master cylinder question--manual VS power

[diesel4me]

My '81 G-10 van,I think needs a master cylinder..it has manual brakes..

The master cylinder on it now doesn't look like the "correct" one ,going by the I.D. chart on A-1 Cardone's site and other listings I've looked up..

The cap on the unit they show has 2 equal sized fluid chambers and has 2 wire bails to secure the cap on it..

The one on the van now has 2 chambers of different size,and only one wire bail to hold the cap on...looks just like the master cylinder on my '82 GMC..

To me,it appears someone might have just put one for power brakes on it before I got the van..

But, I'm not sure if the brake pedal push rod the manual master uses,would snap into the power brake master's piston..I'm thinking it might..the power brake master has the "deep hole" in the piston..the push rod just has a plastic "ring" on its shaft that snaps into that piston hole..

I don't think the push rod could fall out anyway,the brake pedal has a travel limiter or stop on it ,it can only go so far back up when you let off it..

Anyone ever try swapping a power brake master for a manual on a van or square body pickup?..

If I have been driving the van for over a dozen years with the "wrong" master on it,I wasn't very aware of it..the pedal effort did seem a bit high,but I just assumed it was "me",because I'm used to having power brakes on all my other vehicles I drove..the van wasn't "scary" to drive,and if I had too,I could lock the wheels in a panic stop OK..

The manual master has a 1" and 1-3/8" bore size for the rear and front brakes..
One for power brakes has a 1-1/8" and 1-9/16" bore sizes..
So,there would be a bit more pedal effort required to stop,if you used the power brake master..the larger the bore,the more effort required..

Since manual brake equipped vans from 1981 to 1984 are the only thing the manual masters fit,they are quite expensive...like 60+ bucks locally..

Where as a power brake master is available at many stores for as low as 15 bucks !..and fits a ton of other GM cars & trucks..

Since the van is not registered,I would rather not invest a lot of money in it--I just want it to have brakes so I could put it back in use should my truck crap out..

I could get the "correct" manual master from Rock Auto for 20-25 bucks plus shipping,but they are "close outs" that have likely sat on shelves for decades..if a power brake version would bolt up and work,I just assume buy one from Autozone just down the street for less...

I might just buy one for power brakes and see if it'll work,I am hoping someone might have tried this before...if it ends up not working,I can always just put the new PB master on my '85 K10 Suburban,its the same part number ..I'm sure that one is toast from sitting too long too..

 

[diesel4me]

Well,I see lots of lurkers,but no posts in reply...

After days of internet browsing,I finally found a master cylinder thats identical to the one on my van...which is original to it as far as I can tell..

Turns out it is one listed for a 1974-80 G-10 ,I guess GM decided to put last years brakes on mine !..(it IS an '81 going by the build date sheet!)..1981-84 use a different looking master,but is the same bore size ,and costs 40 bucks more..

Also,the best part is--I can order one locally at Autozone for $20 with the core exchanged,but they'll have to get it from their warehouse,which is usually "next day"....glad I kept poking around different online catalogs !..

For anyone interested,the power brake version master cylinder does have the "shallow piston hole",so the manual brake push rod wont work with it--the cylinder would bolt up though,but it also has a larger bore,so even if the pedal rod worked,it would increase the pedal effort a lot..

I was also looking into using a 67-74 Corvette master cylinder for manual brakes,those can be had cheap local too,for about 25-30 bucks..they have a 1" bore also--but I cant determine if they are a "stepped bore" design like my van uses,and I don't want to take the chance it wont act right--it would bolt right up perfectly however,even the line ports are the same sizes..
I see online that many hot rodders use the Vette master when they cant fit a power booster or want to ditch it to fit a larger engine in a hot rod..

 

[82355]

Not a lot of manual brakes out there.

Martin

 

[diesel4me]

Yes,and that's probably why the master cylinders for most vehicles that have manual brakes are more expensive..lack of cores,low demand,etc..

My van is probably one of 10 with manual brakes built in '81....
It was a "base" model with a 250 six,TH350 w/lockup,and it has the single round headlamps,unlike most after '80 that had the quad headlamps or 2 square ones..no A/C,or power windows..guess I'm lucky it had power steering!....
It has a '73 Malibu 307 in it now..(kind of regret yanking the 250 out,it was a good runner ,just gutless,so is the 307 though,maybe worse!)..

I found one parts listing that shows a "heavy duty manual brakes,6000 GVW,that has a 1-1/8" bore master cyl..that one was available only in Wagner brand and was close to 100 bucks..

The van was built in December 1980,so it is an '81,but evidently they used '80 brake parts on it..
I swapped a different rear end that had the slightly larger brakes,and new wheel cylinders that might be slightly bigger bore, but it did not affect the braking any..improved it if anything..so I'll stick with the 1" bore master that is factory on it...

I guess I'll order the master for it so I can get the brakes done--then I'll have to move on to the '85 Suburban,and decide whether to keep it and fix it--or give up on it and part it out or sell it..

 

[diesel4me]

Well,I got the replacement master cylinder today,bench bled it,and installed it..

It wasn't fun,it is more work doing a manual brake version..trying to get myself up under the dash was quite painful..but that was the "easy" part..

I was very frustrated when I still could not get one drop of fluid out of the rear wheel cylinders--put the bench bleeder hoses back on the master and it seemed the front brake section kept making bubbles,and I had bled it on the bench in a vise the first time,because the van is jacked up about 2 feet off the ground in the rear..but the front brakes were applying fine,regardless of the stubborn air bubbles..

After trying to bleed the rears again and still getting not one drop of fluid to the wheel cylinders,and the pedal feeling "dead" the first 2" of travel,exactly as it did with the old master--I decided to try blowing 120 psi of compressed air thru the new rear line I had installed,by sticking my air gun in the rear line coming off the master cylinder..NO air would pass !--even with both rear bleeder screws out--no fluid...

I decided to take off the brake lines at the master cyl.and plug the ports..

Nice hard pedal--so according to the bleeding instructions,that proves the master is bled ok,and is NOT at fault...so at least I could rule a faulty rebuilt master out..


So,I decided to take the new rear hard line off,at the rubber rear brake hose..

A gusher ensued,and right away I figured the dam rear rubber hose had somehow sealed itself up!..

I took the hard lines off the rear hose to the wheels and sure enough--it was plugged solid !..evidently after almost 35 years ,it decided to clog up inside--it still looks nice on the outside though--no cracks,or rot on the fittings..

I had a "good used" stock rear hose off my pickup I kept when a friend gave me a steel braided rear hose he couldn't use on his lifted pickup,that happened to be identical to the one on the van..

I fought with the bolt that holds the hose to the bracket for over a half hour,it was rounded off,and vise grips just kept stripping it worse--finally got it off with a pipe wrench..

Had to "save" that boogered bolt because its a special shouldered one,so I ended up brazing a larger nut over what was left of the head..
Also had a battle getting the rear hard lines to screw into the brass T fitting..had to clean up the first few threads with a tap to get them in..

Then the hard line to the left rear wheel cylinder starts weeping...

It was like new,except for one spot where it clamps to that tab on the rear axle tube..learned the hard way to never put a slit rubber hose over the line to "protect" it from rubbing metal to metal,it only encourages rot!...replaced that line with new tubing,and
I was finally able to get fluid to the rear cylinders and bleed the frigging brakes,6 hours later......and they actually work !..raise the frigging flag!..

This is a typical day,whenever I try to accomplish anything..a job that should take an hour at most always turns into an all day fiasco,and I end up beat and sore for weeks..I'll be laid up after this job for a long time..

My original master was probably still "good",but with 180K on it it was likely not long for this world anyway,and I took it apart before I used it for a core just to see what it looked like inside--the cups were rock hard,the secondary piston refused to come out at all,and the fluid chambers casting had rusted so badly I doubt the thing would have lasted for long even if it was able to work after sitting 12 years..for 20 bucks its worth ditching it..

Now the debate whether to buy a new rear hose,and keep going as far as replacing hard lines,or just leaving it as-is for now is the current situation....I put new front hoses on it sometime in the 90's,they still look ok,but who knows what they are like inside..(the front hoses had become blocked and the front brakes refused to release back then,and I put calipers on it thinking they were at fault,and it was the rubber hoses!)..

I stood on the pedal pretty hard ,the remaining lines still look decent,none popped --most are covered in oily slime from the previous engine,which preserved them--externally anyway...I think I'll hold off spending any more money or effort fixing things till I decide if and when I'll put it back on the road..
I'm sure if I do register it,the first few weeks will be "fun" when all the things that went sour from sitting start failing....

So,live and learn--I would never had thought the rear brake hose could cause the master cylinder to seem "bad"..

Brake line replacement has to be the suckiest job under a vehicle lying on the ground..I hate electrical problems too,but I'd rather fool with wires any day..