Brake question

Correct.

 

It stems from the volume of fluid needed to actuate the piston or pistons on each brake unit. The rear drum brakes only needed a small amount of volume for the small wheel cylinders, as compared to the large single pistons of the calipers up front, so that chamber was much larger. Now, Hillbilly, correct me if I'm wrong, but didn't the 4-wheel disc-equipped sedans have equal-sized chamber master cylinders?

 

Yes they did as a rule. I have seen some exceptions with a small chamber in the master cylinder for the rear discs = Granada Ghias, and a couple Torino Elites. Small piston rear calipers is how they got by with that.

 

Got it.

 

Clark, just to clarify. You said the MC has a smaller front brake fluid chamber as compared to the rear, that would suggest that the MC is for a disc/drum application. Lincolns, Big Mercury's and T-birds that came with 4 wheel discs had MC's with dual fluid chambers having about the same size each. As you probably already know, there are internal differences between a disc/disc and disc/drum MC as well as disc/disc and disc/drum proportioning valve, mixing these hydraulic parts within the brake system can still allow the brakes to "work" but at a reduced effectiveness rate.

 

Good question. I had that Lincoln Mark IV rear disk brake upgrade upgrade done in 2012. My records show that I replaced the master cylinder at the same time but I no longer know if I changed master cylinder models or not.

I would change out the master cylinder as suggested aquartlow if I knew what one to use for the swap. Any suggestion?

 

A full size Ford, Lincoln or Mercury vehicle from the 70's with 4 wheel disc brakes will have a master cylinder that looks like this example: 1975 Lincoln Mark IV Brake Master Cylinder - A1 Cardone 10-1603 (stockwiseauto.com)and it will have a 1 1/8" bore. If you have a local Autozone, this one will(or rather should) work correctly: Duralast New Brake Master Cylinder NM1603 (autozone.com. Hope this helps

 

Thanks aquartlow. Got one ordered.

 

Your very welcome and always glad to help if possible. If your proportioning valve is for a disc/drum brake system, that will possibly need changed as well, but I wouldn't change anything until you install and test the new master cylinder. If you continue having brake issues, especially if the rear brakes aren't doing their fair share, you could always remove the original prop valve install/plumb an aftermarket/universal 4 wheel disc brake proportioning valve based on the GM design. I use one on my Ranchero as well as my late son's lifted F150 4x4, installing it and a disc/disc master cylinder made a huge difference in his truck's braking performance. Keep us posted and good luck with your install

 

I'm pretty sure the proportioning valve was swapped for the 1976 Lincoln Mark IV unit.

So what is the best way to change out a master cylinder? As a teenager in Dad's shop, he used to bleed the lines at the master cylinder connections - while I pumped the brake pedal.

The alternative I suppose is to hook up the lines and vacuum bleed till all the lines are clear - which isn't a bad idea anyway to insure fresh brake fluid in the system.

'Course now I'll probably have to do this by myself.

 

I've done it myself. You need a dowel, a pipe, something to go between the front of the seat bottom and brake pedal. First, fill and bench bleed the new MC. Then, remove the old one, install the new one. Put a big was of rags in a plastic bag, then stuff the full bag under the MC with the opening toward the MC. Make sure that bag doesn't leak. Anyway, crack open the line fittings just barely, enough to let a little trickle of fluid begin to come out. Then use the selected push tool, very slowly push the brake pedal down till you can carefully secure it against the seat without it slipping or going backward. Close both fittings, pull the tool, make sure the pedal returns fully. Then repeat, as many times as is necessary for a firmish pedal. Remember, four-wheel discs are not as firm as all-drum or disc/drum brakes.

 

Thanks handy-andy. I'll try that first.

 

Yah velcome.

 

So I've been through two vendors now for Lincoln Mark IV (front and rear disk brakes) master cylinders including the one posted by aquartlow for AutoZone. In each case, the pictures show equal sized master cylinder reservoirs but both have come with unequal sized just like my current master cylinder. I have questioned the vendors in each case and they swear these are the correct master cylinders. I have no idea if these have equal sized pistons for the front and rear brakes or not. So I give up and I will install the AutoZone regardless.

 

The pictures DO show the reservoirs to be slightly different in size (with the rear one being smaller) - take another look at them from the side.

 

Well the pictures show equal size reservoirs BUT that's not what they ship. Again, when questioned, the vendors swear these are correct anyway. Even the part numbers are correct.

 

Part numbers can be shuffled. What parts houses are you going through?

 

Summit Racing and AutoZone

 

OEM PN for 76 Mk IV master cylinder is D6VZ-2140-A. Also good for 75 Mk IV and 77-79 Mk V.

You might not find an NOS MC, but the original PN might help you get the proper aftermarket part.