Master Cylinder Help

[1972Blaze]

When I got my truck it had the master cylinder in the picture that is off of the booster. When I redid my brakes I used the recommended cylinder that is currently installed in the photo. I do not have enough braking power. I feel as though the master cylinder may be the issue. I would appreciate any thoughts on this. Also, can anyone help me identify the part number or a replacement for the master cylinder that I took off? Both cylinders have the same sized piston (1.125”). The truck is a 1972 with drum rear and disc front brakes. Thanks.

[jradford]

Check for a vacuum leak. Did you reinstall the rubber seal between the master and booster? Had the same problem on my 72. The brake guy had left out the rubber seal and there was a vacuum leak between the master and booster.

[demian5]

I think the one on the bottom is the right one. The other is a car application.

[jradford]

My truck is a 72 as I said and I had the one you have installed and replaced it with the same. Works like a charm once the vacuum leak was fixed. I bought my replacement from Rock Auto and their website sent me the right one. I used a Raybestos. You could look it up and compare the part number to what you just installed. If the piston size is the same then the only difference between the two are reservoir size.

[1972Blaze]

I have looked for a vacuum leak, but not around the master cylinder. I do not recall what is between the booster and cylinder. Is it an o ring or a gasket? I do not remember one when I pulled the old one off. I do have about 16.5" of vacuum when I measure it at the manifold.

Is there any reason to believe that the master cylinder with the larger front reservoir would contribute to greater stopping power at the front brakes?

Thanks.

[jradford]

The seal is a large rubber seal that looks like an inverted cup. The reservoir size should make no difference. Check all around the area between the master cylinder and booster. Pop the hose off of the booster and put your thumb over it to make sure you don't have a cracked hose or bad fitting. Pretty sure it is a vacuum leak somewhere.

[RichardJ]

There is no vacuum between the MC and the booster. Some boosters even have a dimple or channel so that in case the MC should leak, the fluid can drain out and not damage the booster.
If you have a vacuum leak in that area, the booster is BAD. Any fluid leaking from the MC would be sucked into the booster, then into the engine.
Any rubber between the MC and booster, will alter the booster pin length adjustment. Anytime either the MC or the booster is replaced, the pin length needs to be checked and adjusted if necessary.

The larger disc reservoir is sized so that it doesn't run out of fluid before the disc pads wear completely down. It just means you need to check fluid level periodically and it also means when you do put new pads on, you'll have too much fluid.

If both MC bores are the same and the booster pin length is correct, then you probably still have air in the lines. Did you pull the Combination Valve Pin out when you bled the front disc calipers? Watch to see if the pin slides out when you have someone apply the brakes.

[jradford]

Simple question. Where does the vacuum hose hook to the booster? Master cylinder side. It may be something other than a vacuum leak but at least check before making this a science project. The only thing you have changed is the master. If you have the old master see if the seal is still in the end of it. I will look to see if I have a picture of the assembly and post it for you.

[jradford]

Send me your email address at radfordjw@gmail.com and I will send you the factory rebuild files with the pictures. I can't figure out how to get it to attach to this forum. We can all have differing opinions but I will just send the excerpt from the factory manual and you can look at the drawings.

[RichardJ]

Click "Go Advanced" below Message Box.

On Advance page, go down to Attach files and click Manage Attachments.

Click Browse and look through your computer for the file. Select file and click "Open".
Click "Upload".

Go up to message box and click, "Preview post".

[jradford]

Thanks not sure what drives this site but doesn't seem to be the easiest site to use for some things.

[jradford]

Maybe the file is too large but it won't load. Search the forum as there is a guy from years ago that posted a ton of factory manuals in PDF form. I downloaded them as well as buying the factory manual. It has all the info you need. There are two styles of boosters. Single and double diaphragm. Mine is single. If you want as I said I will send the info to you if you give me an email address.