chip-powell

Yesterday evening, my wife and I decided to go out for a little cruise. It did not take long to notice that my brakes were almost non existent. There was no shudder, no noise, it was just that the brake peddle went almost to the floor before even remotely starting to work.


Well, we limped home and I took a look this morning. There are no line leaks. But when I was under the hood, I could hear a whistling sound. It took a few minutes of looking but it turned out to be the master cylinder. The 2 holes on either side of the bottom of the master cylinder were sucking air like you would not believe. I am assuming that this is the culprit.


Question. Since this is a '71 cutlass and this is the original master cylinder, should I just replace it, or (since I'm there anyway) should I replace the power brake booster also.

Koda

I have had that happen to me. Your master cylinder, as you have deduced, is toast. Brake fluid is being pushed past your seals and barfing out onto the booster, as you see there. When you pump it, it goes there, not your calipers and drums. It's the single failure point in the dual brake system. Also, your brake light will not illuminate, because it is designed to see pressure difference in the front and rear legs, and 0 to 0 is not a difference.

If you want the car back on the road, just replace the master with an Autozone one and bleed. Keep the original. You can then rebuild the original at your leisure, maybe send it someplace for cosmetic restoration, should you want that. The booster is probably fine, but you could swap it out too if you want it beautified and rebuilt too with your master.

I am running replacement boosters and MCs on both mine and will restore the originals and put them back on during the restorations.

Fun71

The "sucking" is coming from the booster, likely due to a ruptured internal diaphragm. If you are not losing brake fluid, don't change the master cylinder; just replace the booster.

edit: Koda posted while I was typing. I am not familiar with a master cylinder failure that would cause a vacuum leak so I may be misunderstanding the description of the problem.

Is the master cylinder losing fluid or not?

Koda

I may be misunderstanding the problem, too.

chip-powell

The front reservoir was only half full this morning. The last time that I had checked it (spring when I took her out of storage) it was full.


When I look around at replacement master cylinders, none of them have the 2 rear holes on either side like mine has. What is their purpose (other than to suck air when the mc is bad)?

Grumbler

If the booster was leaking and losing vacuum the pedal would be harder to depress, would it not? The OP says the pedal goes to the floor, sounds like it's losing fluid
Personally I'd do both now, you'll end up replacing whatever part you don't sooner or later

Koda

If the booster craps out, you'll still have brakes. The booster is an assist on the separate master cylinder bolted on the front. If there is fluid loss, the brakes have a problem, and if there's no brakes, there's also a problem. Now, the booster may be sucking up fluid into it, in which case that's two problems.

Pull em both, save the originals, use parts store replacements to get it on the road, restore the originals at your preference later on. I like the little Mity Vac vacuum bleeders.

Fun71

Yes, you are correct. I was thinking that after I posted (had that failure before).

I agree. From the description of fluid loss and sucking sounds maybe both the booster and master cylinder have failed. Back in the early 90s I had a master cylinder that leaked fluid out the back into the booster. I changed the master and after a while the booster failed, likely due to all the brake fluid in there.

jfb

hey buddie I agree do both. I would also suggest for a few days before for knuckle saving and a few choice words to pb power blast the nuts on the firewall and then detail the area you might not have the opportunity again. I had the same problem a whistle and it was the brake booster but I did both because my plan was to put all new parts on or restore everything eventually. I took the opportunity to renew the firewall also and the nuts on the brake booster can be a bear period soak them everyday for 2 to 4 days at least. also get the brake rod correct the length should be measured before removal count the threads and mark it down. good luck and use American made parts also. check auto g--k etc and ask for American made parts I did recently for my steering column rack set up it is new and from Detroit.

D. Yaros

Had this happen to me on my 62 Dynamic 88. No pedal at all. The booster had failed and indeed sucked to fluid out of the MC. Had to have both rebuilt.

droldsmorland

Do yourself a favor and get the booster and master remaned. The seal between the master and booster goes too. Thats just a vac leak though. The fluid loss is the master as mentioned. Freshen both up for safety and do a good four wheel brake bleed. Get a power bleeder. Brake fluid should changed and bled out every 3 years or so on our pleasure vehicles that do a lot of sitting. Your daily should get a similar treatment as well to keep things happy for many years.

chip-powell

Thanks for all of the info guys. I'll hopefully have some time to replace both this upcoming weekend.