Coming back from a nice ride today the brakes were not working as well as normal. When I started to back the car into the garage the brakes went out. No damage to anything and it was nice for the brakes to give out in the driveway and not 50 miles from home.
When I press the brake pedal down I hear a hiss of air in the car. I am thinking the rear vacuum seal has let go. Do I replace the whole master cylinder or have the one I have rebuilt? Can you get the stuff to rebuild it yourself?
Car 71 cutlass with power brakes and front discs.
Delco Moraine Power booster

Thanks Larry

 

When you say the brakes went out what happened?, if the pedal went to the floor you have hydraulic issues, if pushing the pedal was like standing on a brick then it's a vacuum problem. The servo and master cylinder are seperate items but connected to each other, the master cylinder is in front of the servo. The air hiss sounds like a vacuum issue but if the pedal travels too far it can allow the servo to let the vacuum escape. If you have a vacuum leak it would affect how the engine runs, particularly when idling.
You can get rebuild kits for both M/C and servo but if the master cylinder is worn I would replace it rather than rebuild it, the extra cost is minimal compared to the potentially catastrophic consequences of brake failure on the highway - next time it might not fail on your driveway.

 

The pedal goes to the floor while I hear the air leak. Stopping power is very weak. When in park the idle does change(becomes rougher) when I engage the brakes.
Larry

 

The pedal going to the floor indicates a hydraulic problem. The hissing is because you're pushed the valve in the brake booster all the way open. That's a symptom, not a cause.

 

I think what Joe is trying to say is, is the mastercylinder full of fluid? Any leaks at the wheels? Is the vacuum line to the power booster intact and connected?

 

Master cylinder is full and no leaks at wheels. I will have to check the vacuum line.
Larry

 

Double checked masterclylinder and lines and no leaks. Vacuum line is hooked up and "looks good". I will get a vacuum gauge and check for sure as my next step.
Larry

 

No matter how the master looks, it may still be bad, and probably is, from your description of the symptoms.

They're cheap.

- Eric

 

One more time, if the pedal goes to the floor, it is NOT a vacuum problem (unless somehow the booster is holding the brakes on, in which case the brakes would be locked).

Eric is correct. Most likely you have an internal leak in the M/C, causing pressure from the front circuit to leak to the rear, or vice versa. Replace the M/C for starters.

 

OK thanks for the info. I will let you know what happens. Rock Auto here I come>
Larry

 

Or your corner parts store.

- Eric

 

One more check you can make, look at the the flexible hoses, they can fail without leaking, balooning out when you push the pedal down and returning to normal looking when the pedal is released, How old are the brake lines?, they deteriorate over time.
It is much more likely the M/C internal seals have failed but check everything anyway.

 

Yeah I checked rock auto's prices and am going to call a couple of parts places in town that I have had good luck with.
My problem is going to be finding time to do it, with my daughter's H.S. grad and vacation. I have a feeling it is going to sit awhile.

Larry

 

Rusty Roger thanks for the idea. I don't think that is it because it happened all of a sudden.
Larry

 

Update- got the new MC installed but broke one of the lines on the install. Rusted Solid so it wouldn't spin. After looking at the lines I will probably just replace all of them.
Should I replace the preportioning valve at the same time? My feeling is if I don't I will be back under the car in a month.
Larry

 

Yeah, the trick with those lines is to use plenty of penetrating oil, preferably KROIL, and preferably letting it soak a couple of days, then breaking them loose with sharp taps from a hammer against a tight-fitting wrench, then once you've cracked it, slow patient rocking back and forth to free up the nut from the tubing, and even then they sometimes break.

I'd probably leave the valve alone if it's working - you'll have loosened up the connections already, and you can always replace it later if you need to.

- Eric

 

Thanks everyone for the information, we had the same problem on Friday afternoon after we got back from having the convertible top motor and lifts replaced, the brakes went out, the pedal goes to the floor, the m/c was low on fluid, we filled it, pumped the brakes, m/c stayed full, no leaks and nothing on the tires or wheels. So your information helped. We will replace the m/c and hope the servo is ok, or do we need to do both at the same time? Thanks again!

 

Just the master should be fine.

Cheap and somewhere between easy and not-too-hard.

- Eric

 

Car is back on the road, also had to replace the power booster along with the M/C and some brake lines. Now it stops as it is supposed too. Just in time for some good top down weather.
Larry

 

Hope you were missed by Irene - flooded roads don't facilitate good braking!

 

The car was safely in a garage durning Irene. The jeep got the duty driving around after Irene.
Larry

 

Hi all, I just read through this post with replies...very informative.

I just picked up my 66 Toronado from the shop where the carb was rebuilt and a new water pump was installed along with coolant flush, cap rotor points etc etc. I left at the time they closed. On the drive home the brakes were mushy and the pedal could go to the floor.

I understand from the posts this is most likely a hydraulic problem and not a vacuum problem. My question is this...

If there was no work on the brakes during this visit and the brakes definitely worked better when I dropped it off what could've happened in the shop to cause the change? Could bad placement of the car-lift pads under the chassis have crushed a line? I'll look for fluid in the morning on the garage floor. Is there any way working on the water pump could've damaged the hydraulics? Maybe its just happenstance but it is quite odd to me.

I plan to take it back tomorrow morning but in the mean time I wanted to garner some opinions as to what the problem might be...

Jeff

 

If the wheels were hanging, could've got one of the rubber lines!
None of them are usually extended that far.

 

Actually, Rickman, that's a good thought.

If the lines were dry, and they stretched farther than usual, the outer layer could have cracked, leaving the inner layer intact (for now), allowing the inner layer to "ballon" a bit with brak application (not necessarily enough to see).

He should be able to tell if he gets under the car and squeezes each hand-width of he line in his fist while some slowly pumps the brakes. Normally, the line pulse very slightly; if they're bad, one spot should expand too much.

- Eric

 

Thanks guys! So basically what you are saying is if the brakes are spongy I should get it looked at asap and probably change the lines all around. The mechanic who rebuilt the carb said it hadn't been touched since new! The water pump was original (car has 66K miles) and so I can assume the brake lines are original and should be changed.

I found out the car had been sitting in a garage for about 15 years only started occasionally. New brake lines here I come!

Jeff

 

If you are driving a car with 46 year old brake lines, you need to change them. Now.

- Eric

 

scaaaaaaaaaaarrrrrrrrrrrrryyyyyyyyyyyyyyyyyyyyy.