How do I tell if the brakes master cylinder is going bad, or what else would cause brake pedal to slowly sink at red lights etc, & occasionally go straight to floor having to pump them when stopping?

Sounds like they need to be bled...yeah, again. Be sure to keep topping off fluid in M/C during process. As you sit with foot on brake the air slowly compresses. Go around all four, starting with the wheel farthest from M/C

Is fluid level ok? Are there any signs of leaks? Oh and please use extreme caution driving it like this. Your and every other driver on the road safety depends on your brakes operating properly.

If your not losing brake fluid then its time for a new master cylinder. If you live near a Advanced Auto Parts store, Google "Advanced Auto Parts coupons" you can save some good money. The master cylinder isn't a hard job but it dose require a little bit of talent. Example. It should be "Bench Blend" to save time when being installed. When you order the Master Cylinder get a quart of #3 brake fluid.

dido to BohoMark's answer

Man somehow I knew this is what I was going to hear. I guess I kinda already knew the answer, I was just hoping I was wrong or it was something simpler. Lol. Hell my window wiper switch/lever got broke off & Autozone & PepBoys wants to charge me $112-$114 for a combo switch to fix it. Good lord man i didnt pay but $500 for the whole truck lol if a window wiper turner on and off lever cost that much I'm almost afraid to hear what a master cylinder cost. Yeee. I only drive it to work and back home. I work about a mile away from home and there is only one redlight where I have to stop or slow down to turn, I know that once is all it takes so believe me, I am prepared to hit the ditch every time I touch the brake pedal. I live in the boonies so there is always plenty of ditch to the right and traffic is slim to none when I go to work. I DO think of the danger and I don't like the problem. That's why I'm here trying to find answers so then I can find someone to fix it which tends to be easier to do when you can tell someone what the problem is so they know what they're getting into before hand haha. Anyway thanks y'all for confirming what I pretty much already knew. Ugh now to find a cheap master cylinder and decent mechanic who won't charge me both arms legs and eyeballs to fix it

You know, there is always the bone yard. That switch is a $10.00-$15.00 item there. The master cylinder is another question. If it's $50.00 new after coupons, go for it. If you have a vocational training school near by have them switch it out for you. Now here is where I take you to the wood shed. There's a little kid somewhere that's in danger coz you have no brakes! My 2 3/4 year old son is half a world away but someone's pride and joy isn't. Your risking life and limb. Not only of yourself but others. PLEASE DO THE RIGHT THING. You will sleep better.

I had a mechanic which is my boss tell me if you have replaced the master cylinder and bled the brakes and it continues to slowly go to the floor when braking at a light then it could be the brake proportioning valve. My husband is having the same issues with his 1990 B2600 4wd and he had it checked and it is not the master cylinder which he has replaced already and we have bled the brakes a couple of times and then he took it Tuffy's and they said they couldn't find anything wrong the brakes seem to be fine to them and the master cylinder was fine. But when we picked it up to take it home my husband said it still wasn't has stiff as it was when we replaced the master cylinder and it still goes to the floor a bit.

I have a similar problem, not always but sometimes when I'm in traffic I'll lightly tap the brakes and you'd think I curb stomped the pedal. Other times I'll be driving and go to slow down and the pedal will travel half the distance to the floor before I even feel the car slowing down and have to repeatedly pump in order to stop in time. It's quite frightening at times. I've already replaced the calipers/rotors/pads...etc. and the fluid in the m/c stays constant. There is a very loud squealing coming from the brakes but I'm assuming that's because I didn't apply grease when I replaced everything. I've bled/serviced all my brakes about a year ago and I know what they SHOULD feel like. Could this be as simple as an adjustment valve or even a seal or do I need to get a whole new m/c? '98 ram 1500 2wd 5.2 front disc/rear drum

When a brake pedal goes to the floor, its caused by one primary problem - air (or some type of contamination) IS getting in the brake system from somewhere. Remember that when bleeding brakes you MUST do it from at least 2 opposite positions. Most modern brake systems are diagonally opposed so that if you have a partial failure you will have a brake for one side front and rear. This is why master cylinders have 2 fluid chambers and a proportioning valve. You bleed the brakes at one position in the front, then the opposite side on the rear. You must keep the master cylinder full during bleeding. A bad master cylinder will usually have low fluid and signs of leaks. Be careful not to leave the cap off for extended periods as the fluid can be contaminated by dust, grime and even air while open., if the cylinder has a good fluid level and is not leaking it usually is not bad. Look for fluid leaks at the hoses, brake lines and calipers/cylinders. It does not take much air in the brake system to cause trouble and it takes patience and a attention to run the problem down.

Years ago, when I was young and eager, I rebuilt a master cylinder on a '66 Chev Impala because the brakes were so spongy. I was quite surprised to discover that the master piston inside was so pitted with holes that brake fluid flowed THROUGH the piston when the brakes were applied. That's one of those things they tell you is never supposed to happen...

IF your pedal is slowly going to the floor (bleeding off) and you can not find any leaks in the system, you have a bad master cylinder. Fluid is leaking thru or around the master cylinder piston.

Master cylender on amazon is about 30 dollars i replaced mine on a 83 s 10 and money is tight for me also

my dads trucks breaks don't work we`ve replaced the master cylinder but it still sinks to the floor its been 5 months and we still don't know the problem we`ve replaced nearly everything and it still does it has anyone out there have a clue on what the problem is .............

Well we have also got limited funds.O'reilys is very reasonable on most parts.Master cylandar is $21.

my driver side front brakes seized u i put on a new brake caliper and brake shoes and the new brake hose .i took it for a ride it was fine but this morning on my way to work it did it again.is the anybody can help me with this problem

it is 2008 Chevy 3500 diesel

It's your master cylinder I'm going threw the same thing rt now they want 44 buck not included the Reservoir and they also want a $20 deposit until they get the old master cylinder back you can get a rebuild kit but you're better off just to get the whole master cylinder kit. The master cylinder itself put a new one on my brake booster went out in my truck and it's totally different than what it's doing now so I bled the brakes I put new brakes all the way around it new rotors all the way around it gravity bleed all the brakes they work on the first break then the second break like you said if you're sitting at a red light for too long the car starts drifting and then you gotta push the pedal harder so you need to get a new master cylinder and they usually don't come with the reservoir you have to get them without the reservoir and put your reservoir on it but you gotta make sure you bench bleed the master cylinder before you put it back together if not you're going to have air bubbles in it and it's still going to do the same thing. Let me know if you put a new master cylinder on yours and it worked out fine if it didn't let me know because on payday I'm going to get a master cylinder and put it on mine and see if it works I've tried everything else from new lines in the front they said that could be collapsing to brakes to bleeding the brakes two new rotors I've tried everything but the master cylinder in a brake booster and the last time the brake booster went out I had no pedal at all I couldn't stop at all now I can stop but like you said at a red light sitting long time it'll start drifting forward and you gotta push the brake on harder. Let me know if that's any help to you or if you put the master cylinder on I would love to know if it worked out for you

Could it be that the the brake vacuum is leaking?

I was at a red light my truck was smoking from the back end of the truck my daughter boyfriend got under the truck as soon as i got home with it and found brake fliud where it had drip on the brakes he said he bet it was the brake caliber gone bad just wondering what you thought