1969cutlazz

I have a 69 cutlass with manual brakes front drums. I'm calling local part stores like autozone and advance and they have power brake booster with master cylinder as a kit. But its says for front disc brakes. Is there a difference? I'm fairly sure I can't just buy a booster and bolt the manual brake master cylinder I have now to it right. Part store also says I can buy the boosters seperate and the master cylinder seperate. They show a master cylinder seperate for drum/drum but they can't tell me if its for manual or power brakes. I just would like to know if I can go buy the one for my car that is put together as a kit but calls for front disc. Any input is much much appreciated.

redoldsman

The master cylinder for disc brakes will not work properly with your drum brakes. I think you can use the master cylinder you have now. There may be a difference in the pushrod connecting to the brake pedal for the manual and power brakes. I will let somebody else address that.

Eddie Hansen

I would think they would be the same from power to manual drums? (btw I do have the original booster off my 69 with power drums if your interested LOL ), anyway I checked over at rock auto and they list this
DORMAN Part # M71277 {Click Info Link for Alternate/OEM Part Numbers18001122, 18008376, 5458522, 5471817, 8124366, J8126736}
w/Manual or Power Drums Brakes; Bore = 1"
$26.79$0.00$26.79

Eddie Hansen

I do not think the dorman part is correct looking though, the cap attachment looks funky LOL but apparently you could use your master cylinder if it is good, you will need new front lines as well as the manual brake lines ones will not fit right, the booster pushed the master a good bit away from the firewall.

dewcrazzy

I just did the conversion 2 months ago and the part store gave me a m/c looking like the one in the pic above and it never worked. we ended up getting the power disk one and it worked fine with power drums on the front. i bought my booster seperate.

1969cutlazz

Ok so I should buy the booster with the front disc master cylinder? Did you feel a nice difference when you did the conversion?

junior supercar

on your 69 with manual drums, the master cylinder is the same even if you convert it to power drums. only thing you'd need to convert to power drums is the brake booster.

redoldsman

All I know is I wound up with a master cylinder for disk brakes on my 72 Cutlass that has drum brakes all around and it would stop but not like it is supposed to. Good luck.

dewcrazzy

mine is a 72 and it all worked fine. when i put the drum m/c on it wouldn't bleed.
chromevalve1.jpg

Eddie Hansen

No what they are saying is just buy the booster and use your existing master cylinder, I also mentioned that your brake lines will probably need to be changed in the front or at least tweaked and rebent to allow for the booster.

joe_padavano

Coming into this thread late, but the factory actually offered a power booster kit to upgrade manual to power brakes as a dealer-installed item. Yes, you can simply add a booster to your existing drum brake master cylinder. Be sure to use the lower of the two holes in the brake pedal when connecting the pushrod from the booster. You will likely need to readjust your brake light switch after that.

The factory lines from the master cylinder to the distribution block may or may not have enough slack in them to rebend them. If not, you'll need to plumb new longer lines. Otherwise, this is a pretty easy upgrade. Of course you'll need to get the large vacuum tap that screws into the intake manifold to provide vacuum to the brake booster.

Note that the M/C pictured above, with the incorrect cap, is simply an aftermarket replacement. It isn't visually correct, but it is functionally correct. All the brakes care about is the diameter of the piston and the available stroke. The cap style is irrelevant from a functional standpoint.

1969cutlazz

Thank you everyone for all your input. You've been a great help. I ordered a booster from Autozone today and Will be installing it tomorrow. Gonna try and use my existing m/c for now. I made and replaced all the brake lines on the car when I restored it a few years back so hopefully I have enough slack to reach. Otherwise I'll just make new. Wish me luck.

Eddie Hansen

Good luck bud it will be fine

1969cutlazz

Ok guys.....just installed my brake booster with my original manual brakes m/c. Attached the rod to the lower hole on the brake pedal. Bled the brakes and both front wheel cylinders just blew out!!!!! The wheel cylinders are practiclly new. So now i have a feeling the m/c is incorrect. Guess I gotta buy the m/c for disc brakes. And replace blown wheel cylinders. I guess it trial and error since no one knows the right answer for sure.

MDchanic

What do you mean, your wheel cylinders "blew out?"

If that happened, you had bad wheel cylinders, not a problem with your master.
There should be NO force on earth strong enough to damage those wheel cylinders.

Be glad that it happened now, at rest, and not while driving, when you suddenly stomped on the pedal with all your strength to avoid hitting a kid who ran into the road!

- Eric

1969cutlazz

I'm having a had time believing that both front wheel cylinders just happened to blow out. They're like a year old. And they both blew at the same time. The master cylinder is for manual brakes. I believe I need a master cylinder for power brakes. I would imagine the m/c for manual brakes has larger bore holes since it has to work harder than power brake master cylinder.

MDchanic

If Joe Padovano says you don't then you don't.

You don't have a clear understanding of hydraulics and of Pascal's Law.

Read this.

The diameter of the master cylinder has nothing to do with the amount of "work" to be done.

A smaller cylinder will produce more leverage, and require a longer piston stroke.
A larger cylinder will produce less leverage, and require a shorter piston stroke.

Power brakes usually use a slightly larger master cylinder diameter because there is power to spare to operate under less leverage, and the shorter stroke gives the driver the impression of greater ease and less work.

- Eric

dewcrazzy

I couldn't even get the original drum one to bleed so I was forced to use the disk m/c and has worked fine.

1969cutlazz

So its just coincidence that both front wheel cylinders blew out the first time I tried to bleed the brakes? Thats a hell of a coincidence. Youre right, apparently I don't understand the the laws and physics of hydrolic brakes. I'm not saying Joe is wrong. He's helped me in the past. I'm just confused why part stores offers 3 different master cylinders. One for power brakes. One for manual brakes and one for power front disc brakes. Id understand if one blew or they were old and worn out. I just don't want to replace the wheel cylinders and the second I press the brake pedal they pop again.

joe_padavano

The diameter of the master cylinder piston for power brakes is sometimes smaller than for manual brakes. This is NOT the case for GM A-body drum brakes. The factory used the same size piston for both manual and power drum brakes.

Again, what do you mean "blew out"? In four decades of working on these cars, I've never seen that happen. Where did the leak come from? The brakes are designed to work with up to 3000 psi line pressure. Even with the booster, you're not even close to that. You have another problem somewhere.

1969cutlazz

It's leaking at the boots on the front wheel cylinders. Like pouring out when the pedal.is applied.

joe_padavano

And just so we're all clear, you DID have the brakes assembled, drums in place, and shoes properly adjusted BEFORE bleeding, correct?

1969cutlazz

Yes sir. Drums were on and brakes adjusted. I was using an automatic vaccuum bleeder first after I bench bled the master. Then my buddy sat in the car and pumped the brakes I saw fluid dripping out from the bottom of the drum and backing plate. I heard it leaking in the drum. Took the wheel off and the drum and had my buddy applied the brake and its leaking from the boot on the wheel cylinder on both sides. I also attached the push rod to the bottom hole on the brake pedal arm like I was told was correct.

MDchanic

Something's wrong with those wheel cylinders.

- Eric

1969cutlazz

It is possible that I mixed up the brake lines from m/c to proportioning valve. The way I have it now is smaller 1/4 inch line on front of m/c to the front of the proportioning valve and the larger line from the rear of the m/c to the rear of the proportioning valve. I think i may have this wrong on the proportioning valve side. Can anyone confirm this? Please and thankyou.

joe_padavano

First, your car doesn't have a proportioning valve. It has a distribution block that does nothing for the pressure. It simply splits the output to the two front wheels and it houses the differential pressure switch that illuminates the BRAKE light on the dash if you have a leak in one half of the system.

Both pistons in the M/C are the same diameter, so it really doesn't matter which goes to the front and which goes to the back in a four wheel drum car. Normally on an A-body the front reservoir feeds the front wheels and the back feeds the rear.

The distribution block has two input ports, one on each side of the electrical contact. One end has a single outlet port; this goes to the rear. The other end has two outlet ports and these go to the front wheels. The factory used different size threads on the flare nuts so you can't mis-connect the lines.

Bottom line is that there is no possible way for you to apply enough pressure to the master cylinder (even with a power booster) to blow out the wheel cylinders unless they were defective from the start. You said you had replaced them. Were they Chinesium replacements, or brand name like Wagner or Raybestos? Did you replace the entire wheel cylinder or rebuild it?

1969cutlazz

Well I just replaced the wheel cylinders and rebled the system. Everything seems to be working fine. I put the old wheel cylinders on my work bench after I got them off the car and peeled off the rubber boots and the pistons and spring just fell out. Obviously they were bad. They were fine till I converted to power brakes. It's crazy they both decided to fail at the same exact time right after installing a booster. Anyways.....everything seems to be working great now. Thanks for all your input and professional advise. I learned a few things over the past 2 days.

joe_padavano

You apparently never hit the brakes hard enough to pop them before. Be thankful this happened now and not during a panic braking situation. Most folks only use a very small fraction of the cornering and braking capability of their vehicles.

redoldsman

I am curious as to which master cylinder you used.

1969cutlazz

I reused the one that was bolted to the firewall before I installed a brake booster. I replaced the m/c back in 2005 when I restored the car. It was master cylinder for drum/drum brakes.