1970s Measuring the hard brake pedal symptom with 1973 Eldorado

[Finndorado]

Hello Guys

Do you know if I decode my measurements correct ?
I have hard brake pedal, braking power is poor.
Last summer the problem was opposite, the pedal floored.
I renewed the master cylender (correct bleeding etc.) and
the pedal flooring was repaired. I noticed the pedal was hard,
but thought it belongs to the thing. This summer I noticed that
sudden force/panic braking was hard, and almost had to take my
pants to the laundry.
I googled about the matter and checked the straight manifold vacuum,
it was ok (green area, about 16-17 in idle, and more with rpm`s).
Cecked the valve, seems to work ok.
Then I was suspecting the booster diaphragm, bought a manual
vacuum pump with gauge and gave vacuum with it straight to the
booster. No vacuum appeared into the gauge, so I made a conclusion
that the booster diaphragm is broken because it sucks air through
the diaphragm and no vacuum can be formed.
Did I make a correct conclusion ?
I did also following booster tests. Engine off, pedal pumping until
hard, keep pressure and engine on, then pedal should drop. It does
slightly, but I suspect it`s because there is "some" vacuum.
I unplugged the hose from booster and blocked the manifold hose with
my thumb. With thumb the engine idle was good, no thumb (=vacuum leak)
the idle was rough. Put the hose back to the booster/check valve, the idle was
not as good as it was when I blocked the manifold hose with my thumb.
I think there is a "big" vacuum leak through the booster diaphragm.
When driving a while the first tranquil braking is almost ok, but the
followings right after need leg force.
A new booster needed ? And where should I buy it, Caddy Daddy maybe ?
Older topics told me that some fixed or new production boosters last
for only few years.
Thanks for sharing your experience !

Mika

[Michael Petti]

If you think the booster is bad, consider rebuilding. You will get a better unit than you could buy new, from China. Power Brake Exchange in Pittsburgh does a great job. Call and talk to Harold.

[Finndorado]

If you think the booster is bad, consider rebuilding. You will get a better unit than you could buy new, from China. Power Brake Exchange in Pittsburgh does a great job. Call and talk to Harold.

Thanks Michael, I will do that !

Mika

[SaltLife97]

I can 100% attest to a booster issue. A vacuum leak will cause your booster to literally make it seem like you have no brakes. I had the exact same problem and I installed a new master cylinder because the old one was rusty anyways. Didn't fix the issue. I was about to buy a new booster until I saw in my shop manual text that an 'Oring is needed between the master cylinder and booster where they mate. Mine either disintegrated or someone previously never replaced it. Caused a massive vacuum leak I couldn't even hear. So I would double check that and see if it is needed too.

[Finndorado]

I can 100% attest to a booster issue. A vacuum leak will cause your booster to literally make it seem like you have no brakes. I had the exact same problem and I installed a new master cylinder because the old one was rusty anyways. Didn't fix the issue. I was about to buy a new booster until I saw in my shop manual text that an 'Oring is needed between the master cylinder and booster where they mate. Mine either disintegrated or someone previously never replaced it. Caused a massive vacuum leak I couldn't even hear. So I would double check that and see if it is needed too.

Well Kristof, that was a major news to me !
Last summer I installed a new master cylinder
(because of pedal flooring) and now I can see a leakage of brake fluid between the master and booster joint.
I had no sealant between them as I thought there is no liquid change between them.
Still I don't understand the need of sealant,
but if you had the exact same issue, I sure will check it. It has been confusing to me
that there is some vacuum assist when I do the
pedal pumping and then start the engine with pedal pressure, and the pedal does go down an inch. Also the rough idle tells me there is a vacuum leak, because when manifold hose closed the idle is fine.
Did you check the booster rod to master clearance, has it got anything to do with this matter, because I didn't ?
Thanks Kristof 👍👍👍

[Finndorado]

I couldn't have a classic car without this
spectacular assistant of this community 😁👍.

[Chopper1942]

Depending on year, make, and model, some applications had either an 0-ring, round or square cut, or a gasket to seal the master cylinder to the brake booster. If you suspect a leak there, with the engine running, you can take a propane torch, turn it on, DO NOT LIGHT IT, and let the propane flow around the joint where the master cylinder attaches to the booster.  If there is a vacuum leak, the engine rpm will increase. You can do the same thing with carb or mass air flow sensor cleaner.

First, you should not have any fluid leaking from between the booster and master cylinder. If there is, remove the master cylinder mounting nuts, and if possible slide the master cylinder off the booster. Check the end of the master cylinder for brake fluid. If ther is fluid, the master cylinder is leaking.

[Finndorado]

Depending on year, make, and model, some applications had either an 0-ring, round or square cut, or a gasket to seal the master cylinder to the brake booster. If you suspect a leak there, with the engine running, you can take a propane torch, turn it on, DO NOT LIGHT IT, and let the propane flow around the joint where the master cylinder attaches to the booster.  If there is a vacuum leak, the engine rpm will increase. You can do the same thing with carb or mass air flow sensor cleaner.

First, you should not have any fluid leaking from between the booster and master cylinder. If there is, remove the master cylinder mounting nuts, and if possible slide the master cylinder off the booster. Check the end of the master cylinder for brake fluid. If ther is fluid, the master cylinder is leaking.

Thanks Larry !

That propane trick I used with other vacuum hoses.
Yes there is a leak, and the master cylinder is one year old... 

[Chopper1942]

Remove the MC and see if there is a groove in the housing where it goes into the booster. If so, get an OE replacement and if not available, get and o-ring that has an OD the same or a little larger as the ID of the booster. Make sure it is fat enough to fill the groove width. When you reinstall the MC, it should have some resistance when you push it in. If the MC does not have an o-ring, make a gasket and install between the booster and the MC Reinstall the MC and start the engine and leak check with the propane.

Good Luck!

[Finndorado]

Remove the MC and see if there is a groove in the housing where it goes into the booster. If so, get an OE replacement and if not available, get and o-ring that has an OD the same or a little larger as the ID of the booster. Make sure it is fat enough to fill the groove width. When you reinstall the MC, it should have some resistance when you push it in. If the MC does not have an o-ring, make a gasket and install between the booster and the MC Reinstall the MC and start the engine and leak check with the propane.

Good Luck!

Thanks again Larry !

The booster had a good black paint on it, but after reneving the MC last summer I can now see there has been a leak just under the MC/Booster joint.
Does that mean my MC is no goog and leaking even it`s new ?
The leak was not wet now, so the fluid might have gone to the manifold and out from the exhaust..
The code is Dorman M76162 and it`s compatible (in the list) with Eldorado 1975 and several other GM car
but not with Eldorado 1973
The whole MC body is also rusty, should I have been painted it, or is it rusty just because of air moisture ?
The cap or pipe connectors are not leaking and I did a clean install with it.
The old MC body shape is a bit different and it was painted black.
I did not notice any seal in the old or new.
When I lifted the new MC cap I noticed the cap rubber was bloated towards the MC cup, does it mean that
the brake fluid is decreased and the rubber is sucked down ?
I didn`t dare to do anything else because the next day I had to drive the car to mechanic for repairing the electrick windows, but he is also going to pay attention to the MC/Booster.
Now I want to be sure to provide him maybe a new MC and want to get a correct one.
One more thing, my hearing is not that good but the mechanic did hear hissing/sucking sound from the MC/Booster, so the issue may be very clear.
But, should we renew the MC at the same time, because what I think is that the missing seal is not to prevent
the brake fluid to leak, but to prevent the manifold vacuum leak ?
In the picture is the new MC.
https://www.dropbox.com/scl/fi/qkq8ffy0qz37r7tj6jcx0/The-new-MC.PNG?rlkey=lh9ey9oblyxwiz2dx4im3mtwi&dl=0

Thanks
Mika

Edit
I found this MC from Dorman pages and it fits my 1973, but no seal is included or offered..

 

[Chopper1942]

Do you still have the old master cylinder? If so, check two things. 1st measure from the mounting flange to the push rod end of the housing. Are they the same? 2nd check the end of the housing for a gasket stuck to it. If not, look inside of the booster and see if the housing seal/gasket is in place on the flange around the opening for the pushrod. If there is not one in place or it is damaged, you need to install a seal/gasket to stop the vacuum leak. If the piston is wet or there is a lot of fluid around the snap ring, the piston is leaking. Also, make sure it is brake fluid and not assembly lube from the rebuild.

[Finndorado]

Do you still have the old master cylinder? If so, check two things. 1st measure from the mounting flange to the push rod end of the housing. Are they the same? 2nd check the end of the housing for a gasket stuck to it. If not, look inside of the booster and see if the housing seal/gasket is in place on the flange around the opening for the pushrod. If there is not one in place or it is damaged, you need to install a seal/gasket to stop the vacuum leak. If the piston is wet or there is a lot of fluid around the snap ring, the piston is leaking. Also, make sure it is brake fluid and not assembly lube from the rebuild.

Ok, Thanks Larry.
I have the old MC and will check these when I get the car back from the mechanic.
I already ordered the booster push rod measurement tool.

[SaltLife97]

Did you check the booster rod to master clearance, has it got anything to do with this matter, because I didn't ?
Thanks Kristof 👍👍👍

I didn't check it honestly because I felt my vehicle is unused enough with its lower miles. So I just replaced it and called it a day!