A couple of brake questions

I have a couple of brake questions. I was driving the other day and when i went to stop, the pedal was soft, went almost to the floor, and the brake light came on. I only thought the light came on when the emergency brake was applied? I knew the pads and rotors needed replacing but I have question about the rotor. I've attached a picture that shows the surface of the rotor. Is that deep grove considered normal wear or is it indicative of a problem elsewhere? The car is pretty heavy because it's a limo conversion and the rotors are probably for a stock sedan. Could that explain the deep wear?


I noticed the master cylinder was low on fluid. I put some in just in case I needed to drive a short distance before I could change the parts. I pumped the pedal to see if it got any stiffer but it doesn't seem like it. I hear what sounds like air every time I pump the pedal. Is that air in the system or possibly some other component which might need replacing?

Thanks

Here's the picture

You did not say what year vehicle you have. Some of the older rotors had a groove around the center that would act as a wear indicator, when the grove is gone, the rotor is too thin. It sounds like you have a brake fluid leak. Inspect all the brake lines, calipers and wheel cylinders for leaking, also look at the back of the master cylinder where it bolts to the booster for signs of leaking. The "air" noise you hear is the vacuum in the booster, some noise is normal, but if its constant and the pedal is hard, then the booster may be leaking.

Sorry, it's a 1974 Pontiac Catalina. If there was a vacuum leak in the booster, would it show when I check for a leak with a vacuum gauge? If so, where do I connect the gauge? I did it not too long go off the manifold vacuum checking for any leakage and it wasn't abnormal.

If there are no external fluid leaks, I would say the master cylinder is bad.

And what about the light being on?

your car would show a low master cylinder level if you haven't checked it for quite awhile- as the brakes wear the disc pistons move out, the rear shoes wear, wheel brake cylinder piston moves out- the fluid is pulled from your master cylinder- you go to far your brake light will come on- could be a sensor in the master cylinder- designed just for this purpose- could be a switch that your brake pedal hits

depending on how tight your system is- and if the caliper pistons return much and rear wheel cylinders return much after you really push the pedal down- you could experience a sloppy brake pedal- not necessarily air in the sytem- and your light would come on when your pedal went to far- safety feature designed to let you know you have issues

I'd not open the brake bleed lines at this time- you are asking for alot more work and problems- bleeding air out of a system properly is quite often a tricky operation-

CAUTION- be carefull of the brake fluid you put in- regular DOT brake fluid is what it's designed for- don't fall for the gimics of the new Synthetics and don't mix real different type fluids- even a full system flush and these new brake fluids designed for these newer vehicles can give surprising results in an older vehicle like yours you most likely won't like- if you've already added quite a different type brake fluid- try to suck it all out of the master cylinder- it's no fun doing a complete brake system flush properly- not for the faint of heart

1st- fill the master cylinder to about 2 inches below the top- (if you fill it to the top and the brake cylinders don't move, you are likely to see fluid run out from under the master cylinder lid) move the vehicle to a perfectly clean place- newspapers down if necessary- without starting the vehicle- pump that brake pedal for all you're worth- really stand on it- if there are no leaks hitting the ground- with a flashlight get where you can see up close that area of each brake where the line goes in- no leaks- get another person to stand on the brakes- pump them up if necessary and hold hard when the pedal stops-
you could have a 2 part master cylinder- one side usually goes down more than the other- pedal can even go down soft till it hits solid- meaning- there is fluid pressure but maybe only the front brakes or the rear

power brake booster is controlled by a fairly large vacuum line coming from your intake manifold- if it was really bad as in broken- you'd definately hear it at idle- doubt you could tell how much vacuum the power booster is holding or loosing- most in those years with quite a few miles/age hissed a bit when the vehicle was turned off- not a real bad thing

a reasonable simple test to see if your power brake booster is working ok is to start the vehicle up- pull off that large vacuum line right at the fitting that goes into the booster- should really hear some serious suction- connect it back up- should stop hearing the serious suction

vacuum gauge on the intake really won't tell you much unless you're really hear the leak- which would be obvious

short of it- you mentioned your brakes were real worn- chances are you don't have a leak- but you might have cylinder pistons that sort of stay out there and don't return back- spongy pedal as quite a bit of brake fluid has to move to push those pistons back out where the pads or shoes contact the rotors or drums- will seem like you have air in the system- but chances are not- without visible leaks

also- that power brake booster that sits behind the master cylinder- big vacuum just increase the brake pedal presure- pedal pushes the rod into it, vacuum multiplies to push master clyinder rod/piston into the valves that gather fluid and send it top each wheel thru a couple or more junctions

believe it or not- some actually remove that booster and install just a master cylinder- just have to push the brake pedal much harder- like the old days

I would first looks for leaks/seaps- if none that you can see- pull the front wheels, rear wheels, drums, see if there's seapage- don't press down on the brake pedal with the rear drums off

depending on how handly you are- you could push back the front calipers, remove the real worn pads, install new pads, work off the rear drums springs- squeeze the cylinder pistons all the way in- carefully, remove the rear shoes, install new shoes, put the rear drums back on

you might experience a real full and over flowing master cylinder- again- not a real bad thing- you want to avoid an empty chamber-

before starting vehicle- push the pedal in as far as it will go- might even hit the floor- again quite ok- release the pedal slowly- press down all the way again- what you are doing is pushing the brake fluid back into the cylinders as they will move out to make the pads/shoes come in contact with the rotors, drums

check and add brake fluid but don't fill to the top
now you want to see if the brake pedal will go down and stop- become hard- now you can start your vehicle- see if the booster is increasing your actually foot/brake pressure

slow test drive is in order and recheck everything
remember- brakes save lives- faulty or not properly brakes and there is an accident- you could be in serious trouble

hope this isn't too long- enough experience in vehicle brakes to just give a mechanics perspective- investigate- determin the proper way to proceed- don't cut corners- plan on plenty of time- expect some problems- remove the parts you expect to replace so as to be able to determin what parts are necessary and are correct- when in doubt- do just one wheel at a time- gives you the other to look at to see if everything is correct

1 other caution- quite a few parts stores have the repair manual- brake rotor minimums- drum minimums- some parts stores will mic your rotors/drums- some turn rotors and drums- but- they can only turn your drums and rotors so much- can't be below the minimum- you need to make the desision based on what is found- not all rotor/drum wear is bad- sometimes new parts wear in just fine- again- your choice- a business may not have that choice to make and will only do what they know is their standard procedure- most good mechanics advise turning your original rotors/drums if they are turnable- some do push new rotors/drums with new pads/shoes- and you really don't need the high dollar brake parts- again your choice

OR you can take your vehicle to Midas or such- stand, watch, listen, ask questions, know what you are getting before you sign or agree and walk away

good luck in whichever way you decide

Wow. I've only been able to skim through your reply so far but I can see it's going to be very useful. Thank you for taking the time to go into such detail. I can't wait to get home where I can really read through it and get to work.

Thanks again

enjoy the read-

Service Manager for Goodyear in Shawnee, Kansas- was more or less the start of my journey - city drivers you know- we needed the tire sales- we needed all brake business- we needed everyone to come back- interesting thing seen everyday- nice brake rotor/drum machine- nice bleeder system- critical was to not to do more on a vehicle than what was necessary- even back then folks tended to be cautious when asking that their car be inspected by a competent person- course then you have the customer who says -"we trust you, do what you think is best"

think you'll enjoy exploring your vehicles brakes- you might even find you don't have a very serious problem - and can fix your brakes yourself

I'm guessing you'll have to find the rear brake adjuster to back off the rear shoes to get the drums off

interesting to see what you have to say- chuckle as you explore- ask if you aren't sure what you are seeing

have a nice weekend