I just picked up a new MC from Inline Tube. They had a couple to choose from, but I went with one that is "close" to original.

Mine's a 71, so it is single bail cover type for a factory power disc setup. It's going to get connected down to a standard combo valve. I've got 2 questions (so far):
1) What's the bore size supposed to be? The notes from the package say it is a 1 inch bore, but the other ones I've seen are 1 1/8". I know an 1/8" isn't much, but since it is the brake MC.......

2) It has bleeders on the MC. I mean right above the ports. Am I supposed to use them to bench bleed the MC or can I just bench bleed it normally (little hoses into the reservoir) and ignore them? I've just never seen that setup before.

Thanks!

-Mark

 

1" bore is for manual disc brakes. 1-1/8" bore is for power disc brakes.

Those bleeder valves are just a bonus to make bench-bleeding a little easier. You can use them if you want when bench-bleeding, or you can use the hose method. Different method, same results.

 

A 1" bore has an area of 0.79 sq in.
A 1⅛" bore has an area of 0.99 sq in.

That makes 1⅛" 20% larger that 1", which is actually a significant difference in leverage.

- Eric

 

So then the MC that inline sent me must be the wrong one...

That is not good news. It is the MBM 7074 master cylinder and according to the notes:
http://www.mbmbrakeboosters.com/inde...emart&Itemid=6

I'm actually surprised that we need the larger bore for power brakes rather than manual ones. Anyone have a part number for a decent MC that is 1 1/8" bore???

 

The rule for leverage in a hydraulic system is a bit counterintuitive.
You're thinking of the bore diameter as being analogous to the length of a lever.
It's not. The length of the piston stroke is.

When you push a hydraulic piston through a cylinder, you are pushing a certain amount of fluid out of the cylinder.
The larger the cylinder diameter, the more fluid you push per inch (or millimeter, or whatever) of stroke. If you push 1" piston ½" through a cylinder, you move 6.47cc's. If you push a 1⅛" piston the same distance, you move 8.11cc's.
Obviously, 8cc's of fluid will move a slave piston a lot farther than 6½cc's will, so that ½" of piston stroke has less leverage through the 1⅛" cylinder than it does through the 1" cylinder.
Put another way, you will have to push the brake pedal farther to get the same effect with the 1" cylinder than you would with the 1⅛" cylinder, which means that the pedal has more leverage through the smaller cylinder than it does through the larger cylinder.

Translated to power vs manual brakes, you can see that if you have a power booster, you can use a cylinder with less leverage, which will cause the car to stop with less pedal movement, because you've got a machine providing some of the pedal pressure. You could use a smaller cylinder with power brakes, too, if you wanted, but you would lose the stroke reduction, which is a part of the "feel" that people pay for when they order power brakes.

Make sense?

- Eric

 

Hey Eric ... yeah I think I get it now. Basically because of the larger bore, your foot moves more fluid per unit of pedal travel. With vacuum assistance from the booster we can still build enough pressure to stop the car and keep pedal travel in limits...

So here's the thing... it seems MBM only offers the one MC7074 and lists it for our cars, even with power discs up front, despite the 1" bore. Why? And do I have any better options?

Thanks.

Mark

 

Why? Because the 1" will work with either, and it's cheaper to make and stock one item than two.

Options? I have no idea. I am completely out of the loop with resto-upgrade items.

- Eric

 

I was under assumption there was only two master cylinders. one for drum (manual and power) and one for disk (manual and power)

In fact the last master I bought for my 72 with drums came with two different rubber seals to bolt to either the firewall or the booster depending.

 

The Parts Manual shows:
71-72 F-85 Drum 18001122 $58.05
71-72 F-85 Power Disc 18000996 (not listed)

which is actually very little help at all...

- Eric

edit: on second thought, for this particular year, this seems to describe the situation pretty well.
Drum brakes were available as power or standard.
Disc brakes were available as power only.
So there's one M/C for discs, one for drums, whether power or not.
Looks like in 1971 / '72, Olds made a single M/C bore for all Cutlass drum brakes, regardless of power.

 

as a side note when I purchased my 72 Supreme (power drums) it had the large square reservoir meant for disk brakes. the car always had poor breaking. I replaced it just last season with the proper oval shaped drum master and it stops just fine now.

 

Ok... I called around and I will need to confirm this with MBM, but it seems that MBM's web site might actually be wrong. I talked with 3 different retailers and all said that the MC7074 it is the 1 1/8" bore MC used for GM 70-74 vehicles with power disc brakes up front and drums in the rear.

One said that "We sell this setup all the time for Chevelles and Goats (sic) doing power disc conversions." I am assuming that GTO's, Chevelles, Skylarks and all A-bodies should be fairly similar. Does anyone have any experience with this setup at all???

@MDchanic: One person asked "what difference does it make", so I read him your notes, almost word-for-word. I think it opened his eyes - pretty funny.

 

 

the proper master should be square shaped NOT oval shaped.

 

return that one and go to your local parts place. they should be able to sell you the proper one. Again, its the 1 1/8" bore square shaped one.

 

I am not exactly sure what you mean. The MBM one is sorta' square-ish / rectangular when looking at it from above. It has two reservoirs in it, divided by a plate extending up from the bottom. The rear reservoir is much smaller than the front one.

Basically, it looks just like the one on my car, except that it has bleeder screws in the MC.

 

Found another link which describes them as having a 1 1/4" bore. Never heard of that size.

http://www.bowtiemuscleparts.com/product/BMPMC-7074

The second pic there of the MC installed on a Camaro is a lot more craziness than I am going to put in. Since mine's a 71, I was going to put in a combo valve mounted just under the MC which will (ok, should) allow my headers to fit in nicely and not boil the brake fluid....

http://www.justpowerbrakes.com/image...x?productId=28

That includes a couple of shots of it... Man I really hope that this is the right MC as it was not cheap and seems much more solid than my original.

 

Drum Brake Master cylinder:




Disc Brake Master Cylinder:



- Eric

 

On W30 4 speed aplications,you could not get power brakes, they were all manual disk in 1970 and 1971. Unlike 1970, which both power and non power disk aplications were the same "EB" single bail 1 1/8 bore, 1971 used a dual bail rounded master on manual disk brake cars.

 

The distributors are correct! I called MBM on this and they told me that all of their documentation on the MC7074 says it is a 1 1/8" bore. Only the website says 1". The rep from MBM told me he would put in a "ticket" to have the website corrected.

Hopefully, they will update it soon.

-Mark

 

Make sense? Not really. Your explanation does explain why manual brake pedal travel is greater than power brake pedal travel. The difference in cyclinder dia. sizes has to do with system "pressure" not volume flow. On manual brakes applied pressure (foot pressure & leverage) divided by cyclinder bore area equals system pressure. The power brake system booster adds to the applied force. Putting a manual 1" MC on a power brake system creates a touchy pedal. Even changing the pedal leverage ratio will not reduce the touchy feel. So GM engineers increased the MC bore to correct the pedal feel.

This site helps explain it: (or check out street rodder sites)
http://www.peterverdonedesigns.com/f...m%20theory.pdf

I've had a 1" MC on power brakes before and the pedal was super touchy. To the point where it was stressfull driving. I'd look for the correct 1-1/8" MC

Don

 

There is a volume of fluid physically moving through the system to cause displacement of the pads / shoes, whose movement varies depending on the relative cylinder sizes. Yes, after the fluid has pushed the pads / shoes into position against the friction surface, and removed any "give" between them, then it is a matter of pressure, but then the pedal pressure is multiplied just as you said (and just as I described) - pressure will be 20% more with a 1" cylinder than with a 1⅛" cylinder.

Which equates to the system not having the right "feel" with the smaller cylinder. True, I mentioned pedal stroke, and not "touchiness," but they are two sides of the same coin - increased stroke leads to a feeling where the position of the pedal in space seems more important than the pressure on it (like a gas or a clutch pedal), and makes it feel "touchy" because we are accustomed to receiving a pressure feedback through the brake pedal, rather than pushing it down a certain distance to get a certain amount of slowing. My mistake - I clearly should have thought through and presented that better.

- Eric

 

Mark, did you get this master cylinder and if so, installed it yet? If so, what do you think of it?
I am in the market for a good repro MC for my 72 Cutlass with pwr discs.

 

Hi Rob - I did. It works very, very well, though anything would've been an improvement from my old one. It is supposedly "close" to the original with the bleeders on the MC. They did come in handy during installation.

It all depends on what you are looking for - I didn't really care that much about the look, but I didn't want anything too chromed up or anything (it would make the rest of the engine compartment look even dirtier.

The various places I called had wildly different prices for the same MBM 7074 master cylinder - so just watch who you get it from, I guess.

 

Glad to hear you had good luck with it. I just ordered the same model from Pirate Jack's Rod & Customs. Time to get that cruddy booster replated now!

 

Please let me know how that works out. I have my old booster still and I wanted to have it rebuilt and replated, but I figured the cost would be WAY too high.

 

Only 155 for a rebuild, replate, and replate of your MC cover.
Not too bad if originality is important. The repros are kinda easy to spot...

 

Sweet. That's an excellent price!!! Please let me know how it works out when you get it back.

I don't care that much about originality in my car. Well, that's not true. But, stopping became really important when I realized I couldn't do it very well. So I settled for a repro. My brakes were really squirrely when I got the car, but the PO told me that, at least.

BUT - I kept the original and now I know I can get it redone for a very reasonable price!!!

 

Hey Scott, where did you have your brake booster rebuilt and replated? Thx.