What size master cylinder?

[Jokester]

Hi friends,  

I'm in a quandry.  My brakes have never been great, so it's time to fix them before all the summer trips.

It's a 33 Chevy sedan w/ mustang independent suspension.  The front and rear brakes are 74 Nova; disc front and drum rear.  The master cylinder is under the floor, not sure what it'd from or bore size.  The booster is about 7" diameter but don't know its origin either.  From the looks of it I'd say salvage yard.  The proportioning valve looks to be  OE from the same Nova donor car.  I'm inclined to trash the master cylinder, prop valve, and booster all together.  The stuff at the wheels all looks to be ok and/or has been replaced.  

I've never been able to lock up any of the brakes.  There have been times (especially going downhill) that I was braking for all I was worth and still wasn't slowing the car down any.  My suspicion is that I'm pretty much using rear brakes only.  I put new rotors on about three years ago and they still look brand new.  

I'm finding lots of choices in the aftermarket, so here are my questions.

What bore size should I use?  OE on the Nova was 1 1/8".  Most of what I have found is 1".  Seems like bigger would push more fluid, yes-no?

Should I be using a residual valve on the discs?  I don't have one now (this may be a big part of my problem), unless it's built into the prop valve.

Do we like the aftermarket adjustable proportioning valves?  or should I go with the OE type?

I appreciate any and all suggestions.  It would be nice to leave real skid marks, not the replica kind.

thanx.

.bjb

[wayne petty]

a few things....

tire sizes front and rear...

distance from the center of the brake pedal pad to the pivot bolt on the brake pedal.. and the distance from the pivot bolt to the MC push rod connection...  center to center measurements..  so brake pedal ratios can be calculated..

how much intake manifold vacuum do you have at warm idle...

how much intake manifold vacuum do you have hot in drive.. at 0 MPH


how much intake manifold vacuum do you have at 25 MPH.

does your power brake booster actually work???

engine warmed up...   shut if off..    pump brake pedal 5 or 6 times..    hold foot with light to medium pressure on the brake pedal... start the engine..   does the brake pedal fall away slightly?

with the engine idling .. depress the brake pedal with light to moderate pressure..   hold foot totally still...  shut engine off..  the pedal should NOT kick upwards..

again with the engine idling...

foot off the brake..  after 45 seconds of idling..  shut off the engine..    wait 10 seconds..   how many pumps of the brake pedal do you get assist.. one full.. usually partially on a second..

[enjenjo]

In effect what you have is a Nova braking system. Not the best, but effective in the original application. So if it's not working as well as it did in the stock application, you have to look at what parts are different.

In a normal brake system the front brakes do most of the work, because of weight bias, and dynamic weight transfer when stopping. This is with all tires roughly the same size. I am assuming that the rear tires in your case are somewhat taller, and somewhat wider than the front ones on your car. This pretty much rules out an adjustable proportioning valve as it reduces rear brakes, and you are not experiencing rear brake lockup.

At the wheels, everything is the same, so we don't need to look there. The combo valve is the same, so that is out too. You say the rear brakes are working well, so it's not likely that a proportioning valve will help, as it can only reduce braking, not add it. We can also rule out a rear residual valve as that seems to be working too.

The stock system had the master cylinder mounted 12" or so above the front caliper. This gave the system some head pressure, which was enough to hold the front pads near the rotors. With you master cylinder under the floor, you have lost the head pressure, which may be allowing the caliper pistons to retract more, delaying front brake application, as there is more fluid needed to fully apply the brake. This usually manifests itself as a noticable delay in effective braking, but the brakes will usually get there. This does not sound like your problem, but a 2 psi residual valve in the front line might help, and won't hurt.

This leaves the master cylinder, booster and pedal as variables. Since the master cylinder is unknown, you should make sure it's the same size as a Nova master cylinder. A picture or casting number will help with identification. Your booster is considerably smaller than the Nova which usually had a 10" single diaphragm booster that would produce roughly 1300 lbs to 1500 lbs pressure under hard braking. A 7" single diaphragm booster will produce 600 lbs to 700 lbs under the same conditions, everything else comparable. A Nova brake pedal, with power brakes had roughly a 5 to 1 brake pedal ratio. I have no idea what your pedal ratio is, but a stock pedal in your car is just over 7 to 1.

So, to review, you are pretty much on target except for possibly pedal ratio, and booster assist. What are our options? A 7" dual diaphragm booster will produce about 900 lbs to 1100 lbs pressure, and takes up about 2 1/2" more length, but no more diameter. If there is room, an 8" dual diaphragm booster will get you to 1100 lbs to 1300 lbs, but needs more room all round. If you have power steering, a Hydroboost will get you back to 1500 lbs, with 2 1/2" more length than the booster you have now, and slightly less diameter. If your pedal ratio is higher than 5 to 1, like 7 to 1, you might be able to get by with just a 7" dual diaphragm booster and no other changes.

On master cylinder bore size, if you go larger, you reduce brake pressure. To simplify, if you apply 100 lbs pressure with your foot, and the master cylinder piston has an area of 1.5 square inches, the brake pressure will be 66 psi. If the area of the master cylinder piston is 2 square inches, the brake pressure will be 50 psi. So if more booster does not get you enough brake, you can fine tune it by going to a slightly smaller bore master cylinder, but at the expense of more brake pedal travel to displace the same amount of fluid.

[Mikej]

Hi friends,  

I'm in a quandry.  My brakes have never been great, so it's time to fix them before all the summer trips.

It's a 33 Chevy sedan w/ mustang independent suspension.  The front and rear brakes are 74 Nova; disc front and drum rear.  The master cylinder is under the floor, not sure what it'd from or bore size.  The booster is about 7" diameter but don't know its origin either.  From the looks of it I'd say salvage yard.  The proportioning valve looks to be  OE from the same Nova donor car.  I'm inclined to trash the master cylinder, prop valve, and booster all together.  The stuff at the wheels all looks to be ok and/or has been replaced.  

I've never been able to lock up any of the brakes.  There have been times (especially going downhill) that I was braking for all I was worth and still wasn't slowing the car down any.  My suspicion is that I'm pretty much using rear brakes only.  I put new rotors on about three years ago and they still look brand new.  

I'm finding lots of choices in the aftermarket, so here are my questions.

What bore size should I use?  OE on the Nova was 1 1/8".  Most of what I have found is 1".  Seems like bigger would push more fluid, yes-no?

Should I be using a residual valve on the discs?  I don't have one now (this may be a big part of my problem), unless it's built into the prop valve.

Do we like the aftermarket adjustable proportioning valves?  or should I go with the OE type?

I appreciate any and all suggestions.  It would be nice to leave real skid marks, not the replica kind.

thanx.

.bjb

Bigger will move more fluid but smaller will give you more pressure.  15/16 They have several articles in the car mags on this.

[Jokester]

Thanks Wayne, I can get that info, but won't be able to post until probably Monday.  I'm pretty sure vacuum isn't the problem.  I don't notice any problems with the vacuum related questions you ask.  The engine is pretty much a stock 327 and is smooth and quiet, probably a stock cam.

Tire size may have an impact.  Fronts are 5.5" wide x 24.5 tall.  Rears are a little wider and approaching 27" tall.  I'm considering taller for the front, but I'd like to stay where I am just because the tires aren't very old and I like the look.

I think Frank is taking the correct track.  I appreciate the info on the booster sizes.  Larger diameter is out of the question.  I'll have to check on the length; the dual 7" may be the ticket.  Right now booster and MC add up to 13".  I may have room to grow there.

It's been a long time ago, but I think the MC is a 1 1/8" re-man unit off of some GM vehicle.  It's installed backwards from normal firewall mount, and the ports face the frame.  I think I can work with ports from either side.  The pedal is likely OE 33 Chev.  The guy I bought it from drove it home in its original form.  There's some pretty clumsy hacking and welding on the MC mount, but it's sturdy.  It would be nearly impossible to change the pedal ratio.  I would have to pull the engine to remove the pedal assembly.  Either that or cut it out with a torch.  So I'm hoping it doesn't come to that.

Sounds like I'll be going to more boost and a front residual valve at the least.  I'll have to decide whether 1" or 1 1/8" is better.   One listing on Ebay indicated that 1" was for 2600 lb and under and 1 1/8" for over that.  Don't know if I believe that or not.

Thanks for the great info.  I'll post again next week after some more checking.

.bjb

[Jokester]

Brake pedal has been modified.  The long arm is 11", short is 2 1/8" for just a little over 5 to 1.  Current bore is 1 1/8" just like the OE disc brake Nova.  I do have room for the longer booster, so I'll definitely be getting the dual diaphragm one.  Old MC is leaking, so it's definitely coming out too.  The brake line from the prop valve follows the transmission crossmember across the car, so it's going to be re-routed around the front like normal cars too.

Don't have time tonight to do the vacuum checks, probably tackle that on Monday night.

Thanks for the help.

.bjb

[wayne petty]

you might want to search through the power brake boosters on this site.. they are also available elsewhere..

http://www.mbmbrakeboosters.com/index.php?option=com_virtuemart&Itemid=6


after the vacuum check...    

you might just want to swap out the master for a smaller bore version..

over the next few nights.. i will try to cross reference the 76 master.. to several that have smaller bores but the same size fittings and depth of piston push rod hole..

i wish i could find a mid 1990s bendix hydraulic brake pictures catalog that actually listed parts by shape and size.. IN GROUPS,  i have seen them..  a PDF file would even be better..  this showed 3 to 6 part numbers for each picture.. listing them by bore size..   man what a brake tuners special..

[wayne petty]

here is a you tube video on booster function...

this i just ran across these automotive training manuals..

its a 17 page PDF file Training manual on how power brake boosters work and how to properly test them..

its important knowledge...

http://www.autoshop101.com/forms/brake05.pdf

in addition..

info on disc brakes...

http://www.autoshop101.com/forms/brake04.pdf

really good section on drum brakes...  including some domestic info..

http://www.autoshop101.com/forms/brake03.pdf


on the master cylinder...  inside.. they all work the same.. so its important to know...

http://www.autoshop101.com/forms/brake02.pdf


the theory of how they work.. and sizes verses apply force...

http://www.autoshop101.com/forms/brake01.pdf


this section on the parking brakes

http://www.autoshop101.com/forms/brake06.pdf

25 pages on diagnosis of the braking issues..

http://www.autoshop101.com/forms/brake07.pdf

proportioning valves.. brake bias... weight transfer changing the braking function..

http://www.autoshop101.com/forms/brake08.pdf

on the ABS brake system... learn how one operates.. learn how they all operate... in most cases..

http://www.autoshop101.com/forms/brake09.pdf

abs diagnostics..  

http://www.autoshop101.com/forms/brake10.pdf


still more ABS theory..

http://www.autoshop101.com/forms/brake11.pdf

traction control

http://www.autoshop101.com/forms/brake12.pdf

brake terms glossary

http://www.autoshop101.com/forms/brake13.pdf

sorry for burying you in articles...

somebody might want to move the links to the tech section..

this is really important info to know to properly understand brakes...

[jaybee]

Sounds like all variables have been accounted for and what you've got left is pedal ratio. You've got a "power brake" pedal ratio with a much less powerful booster than stock, so you have trouble getting enough pressure at the wheels to do the job. A smaller m/c would be the easiest swap...but your pedal will travel a little farther to move the same amount of fluid so make sure you have sufficient clearance between floor and pedal to accomplish that. A dual booster or hydroboost would help without increasing your pedal travel.

[Jokester]

Parts are on the way to me.

New question.  Which goes closer to the master cylinder, the residual pressure valve or the proportioning valve?

Also, can the proportioning valve be mounted with the adjuster knob facing down?

thanks again for all the input, it's been really helpful.

.bjb