Master Cylinder bad?? 71 Van Brakes Fading

[ndemge]

Have an old '71 1ton Chevy Van... <br /><br />Recently rebuilt the 350<br /><br />Driving it to work this morning, the brakes are there sometimes, sometimes not.<br /><br />Sometimes it's a nice firm pedal.<br /><br />Other times, pedal goes almost to the floor, brake light comes on... when this happens, I will get SOME braking, but not much. pump it a few times and it will come back.<br /><br />Would this be a bad master cylinder? Drove it in the 40 miles to work today, autozone is only a few miles away, thinking about fixing before driving home, really had to keep my distance on the way in not knowing when they would work good, or just slow me down.<br /><br />thanks.

 

[DJ]

Re: Master Cylinder bad?? 71 Van Brakes Fading

ndemge,<br /><br />Don't fool around with brakes! <br /><br />It probably is the master cylinder, or you've sprung a leak.<br /><br />Remember, if you change the master, you have to bleed the system.

 

[Bondo]

Re: Master Cylinder bad?? 71 Van Brakes Fading

Yep,<br />Sure Sounds like it......<br />Go out,+ Check the Fluid Level....<br />If it's Still Full, I'd Bet on the Master being Gone......<br />If it's Down, Look for a Leak......<br /><br />DJ's Absolutly Right.......<br />This is NOT a parking lot repair.....<br />You'll have to bench bleed the master, <br />Then Bleed the Whole System.......<br />right rear wheel 1st...<br />left rear wheel 2nd......<br />right front wheel 3rd....<br />left front Last......<br /><br />I'm Betting that on a 1971, You're going to have a B!tching time Loosening the Bleeders... Rust...<br />Hit them with penertrating oil Now... It'll be Easier Later.....

 

[ndemge]

Re: Master Cylinder bad?? 71 Van Brakes Fading

it's a bit down, but not that far.<br /><br />the intermitant fading is what made me think MC.... could a leak have this affect? and how far low on fluid will cause this ?

 

[NOSLEEP]

Re: Master Cylinder bad?? 71 Van Brakes Fading

Your master cylinder is leaking past the <br />primary valve. If you bleed the new master cylinder<br />before installing it, you wont have to bleed<br />the wheel cylinders after.<br /><br />Or your brakes are far out of adjustment.

 

[Bondo]

Re: Master Cylinder bad?? 71 Van Brakes Fading

If it's only down a little, it's Proabably the MC.... <br />If it's a leak, pumping will bring the pedal up, but it'll continue to drop as long as pressure is held on it....And, Fluid will be Gone with enough Time.....<br />It Sounds like it's By-passing Internally....<br />A New MC would be the answer there.....<br /><br />Low Fluid doesn't Cause it, It's the Results of a leak......

 

[ndemge]

Re: Master Cylinder bad?? 71 Van Brakes Fading

gotcha.<br /><br />so,... how would you go about changing it out in the parking lot so I wouldn't have to bleed all the brakes?<br /><br />luckely, we have some tools here... and a company van I can use to go get parts (and get my butt home if I have no other choice)

 

[Bob in Calif.]

Re: Master Cylinder bad?? 71 Van Brakes Fading

Assuming you have power brakes, be sure and check the bottom of the "booster" housing, which the master cylinder is bolted to. If the cylinder is leaking, there is a chance the missing fluid is resting in the bottom of the housing.<br /><br />The brake fluid can attack the rubber booster bellows and cause it to decay, causing a vaccum leak. Hopefully the bellows isn't damaged, as the power brake unit can be quite pricey.<br /><br />Also, there is a good chance your brakes wont be really safe unless you bleed the wheel cyinders after replacing the master cylinder.<br /><br />When you bleed the brakes, start with the one farthest from the master cylinder....right rear, reft rear, right front, left front.<br /><br />Make sure you bleed the wheel cyls. a couple of times each, so no more air is present and the fluid is clear. Lastly, make sure you check the fluid level in the master cyl. after you bleed each cyl., or you will suck air in to the system and have to start over.<br /><br />If you have front disc brakes, they will use the larger of the two wells in the master cylinder.<br /><br /> ...Bob in Calif..

 

[NOSLEEP]

Re: Master Cylinder bad?? 71 Van Brakes Fading

You need to put the cylinder into a vise.<br />Fill the reservoir with fluid and cover the two<br />brake lines with you fingers. Pump the cylinder<br />with a socket extension or screw driver and bleed<br />the two brake lines the same way you would the<br />brake cylinders. Then install the master cylinder<br />and bleed the lines again at the m/cylinder when<br />you have hooked up the brake lines buy cracking <br />them loose at the M/cylinder one line at a time<br />the same way you would the wheel cylinders.<br /><br />you could do it by installing the cylinder first <br />and bleeding the m/cylinder before installing the<br />two break lines. as described above.

 

[NOSLEEP]

Re: Master Cylinder bad?? 71 Van Brakes Fading

If you do it the way I describe there is no need <br />to bleed at the wheel cylinders or calipers as the<br />air in the line will hydraulic at the m/cylinder<br />and will be forced out at the brake lines <br />connected to the mastercylinder.<br />I have done many in my day that way.

 

[rwise]

Re: Master Cylinder bad?? 71 Van Brakes Fading

I do mine much like NOSLEEP is saying, works for me, never had a problem with it.<br />Richard

 

[ndemge]

Re: Master Cylinder bad?? 71 Van Brakes Fading

I'm going to be going with the nosleep method, as I am at work, by myself... kinda hard to bleed the brakes. if I do it at the MC, I should be able to do it from there and not use the pedal.

 

[NOSLEEP]

Re: Master Cylinder bad?? 71 Van Brakes Fading

You will still need someone to pump the brakes<br />for you while you are bleeding the lines.<br />To bleed the lines. Pump the pedal a few times<br />and hold it down, then crack the line loose.<br />then tighten line, and repeat the proses until<br />no more air comes out. Dont forget to make sure<br />the reservoir has fluid in it at all times.<br />If you are alone, then you could static bleed<br />the lines by cracking the bleed screws loose at<br />the wheel cylinders and letting the fluid run<br />out. But that can be very messy and time consuming<br />and doesn't always work as well.

 

[ndemge]

Re: Master Cylinder bad?? 71 Van Brakes Fading

drove the work van home last night. ... stopped and got a new MC and took it home to bench bleed it. (came with nift kit w/ hoses and plugs)<br /><br />Boss man was here today to pump the brakes for me and had it on and ready to go in about 20 minutes.<br /><br />Drove it to autozone to return my core, did great.<br /><br />Thanks for the help everyone.