disc brake questions

[makovai]

I just converted my 1972 blazer to 4 wheel disc brakes. My big questions are this:
I tubbed a corvette with a D60 in the rear and put drums on it. when i did this i had to change the proportioning valve out to a 70's style valve for front disc rear drum. It worked great. Why if i changed my blazer to 4 wheel disc brakes would i not have to change it to like a corvette valve for 4 wheel discs?

Are the 2 different resevoirs in mastercylinder completely seperate? In other words could i not even put rear breaks in at all, fill the front resevoir and have full working front brakes? can any air get from the front lines to the rear lines or vice versa?

 

[Hogback Fabrication]

On a K5, the front and rear brake systems have different partitions in the master cylinder reservoir. So yes, if you had no fluid in the rear brakes you would still have front brakes (or vice versa).

Swapping master cylinders and/or adding a proportioning valve seems to be different for everyone. It really depends on your truck and how well your brake system works. I have 4 wheel discs with my stock master cylinder and it seems pretty much perfect.

If your brakes feel good and all 4 seem to lock up at about the same time then run it.

 

[makovai]

On a K5, the front and rear brake systems have different partitions in the master cylinder reservoir. So yes, if you had no fluid in the rear brakes you would still have front brakes (or vice versa).

Swapping master cylinders and/or adding a proportioning valve seems to be different for everyone. It really depends on your truck and how well your brake system works. I have 4 wheel discs with my stock master cylinder and it seems pretty much perfect.

If your brakes feel good and all 4 seem to lock up at about the same time then run it.

Thanks.
No........ i dont have any brakes, and thats my issue. I new the rear was messed up because i have a bad caliper. But i cant figure out why i have no front brakes. They worked fine in my 6 lug setup. I didn't change anything. I blead the crap out of them and they work if i pump them 10 times or so but only for a second. Then they are all spongy again. I'm very frustrated. There is absolutely no leaks in the front calipers and they dont work.

Any ideas?

 

[4X4HIGH]

Sounds like a bad master cylinder to me. It is bleeding down within itself.

 

[makovai]

Sounds like a bad master cylinder to me. It is bleeding down within itself.

]

Figures i just replaced the booster just before the 3/4 ton conversion. funny it worked up to the day i tore it apart though.
Bad mastercylinder has crossed my mind this evening as well.

 

[Hogback Fabrication]

I will second a vote for the master cylinder. Sometimes stirring up old fluid and working the master cylinder a bunch (bleeding it) can kill it, especially if it was marginal to begin with.

You should feel lucky that a master cylinder for your truck is cheap and easy to replace

 

[makovai]

I will second a vote for the master cylinder. Sometimes stirring up old fluid and working the master cylinder a bunch (bleeding it) can kill it, especially if it was marginal to begin with.

You should feel lucky that a master cylinder for your truck is cheap and easy to replace

And of coarse for weeks theres been an ad on craigslist for a booster cylinder complete setup that was brand new for $75 and i cant find it.

Maybe i should be thinking about 1 ton setup since i have to replace it anyway.

 

[ntsqd]

If you still have the stock combo valve, has the shuttle pin moved from center?

 

[makovai]

If you still have the stock combo valve, has the shuttle pin moved from center?

If it did, it did it by itself. I haven't adjusted anything on the brakes.

 

[ntsqd]

If it did, it did it by itself. I haven't adjusted anything on the brakes.

Can happen during the bleeding process.

 

[Leper]

You can get a disc/disc prop valve from gmpartsdirect.com for under 120.00 shipped.


Something many people forget about when bleeding brakes with an old MC is the stroke. You need to put a block of wood or something below the pedal or at least be sure not to push the pedal down very far. Think about this... As you use your brakes on a daily basis, the pedal moves about the same amount every time you push it. The shaft that goes through the seal on the MC goes in just so far. Fast forward a year or so. The shaft now has corrosion/junk/pitting/rust on the part that is always outside of the MC seal. Now, when you go to bleed the brakes, you are pushing the pedal down as far as you can. The rough part of the shaft is now being forced through the seal of the MC. You might as well have wrapped it in sandpaper and shoved it in. To get a visual of what I'm talking about, go look at a shock absorber shaft on a junkyard truck. It operates the same way. Now imagine that shaft going through a seal on the shock and you will understand. I have heard of people that have never limited MC travel and never had a problem. It has happened to me once,(only once because I use a block now) and working in a dealership for the better part of 20 years, I have seen it happen to others.

Hope this helps.
Leper

 

[makovai]

You can get a disc/disc prop valve from gmpartsdirect.com for under 120.00 shipped.


Something many people forget about when bleeding brakes with an old MC is the stroke. You need to put a block of wood or something below the pedal or at least be sure not to push the pedal down very far. Think about this... As you use your brakes on a daily basis, the pedal moves about the same amount every time you push it. The shaft that goes through the seal on the MC goes in just so far. Fast forward a year or so. The shaft now has corrosion/junk/pitting/rust on the part that is always outside of the MC seal. Now, when you go to bleed the brakes, you are pushing the pedal down as far as you can. The rough part of the shaft is now being forced through the seal of the MC. You might as well have wrapped it in sandpaper and shoved it in. To get a visual of what I'm talking about, go look at a shock absorber shaft on a junkyard truck. It operates the same way. Now imagine that shaft going through a seal on the shock and you will understand. I have heard of people that have never limited MC travel and never had a problem. It has happened to me once,(only once because I use a block now) and working in a dealership for the better part of 20 years, I have seen it happen to others.

Hope this helps.
Leper

Well, i will say it is a very very valid point. I never really thought about that and it makes alot of sence.

 

[ntsqd]

I lost the M/C on my '91 about 2 months ago to that very thing. Was only bleeding it after swapping in the 14bff.

 

[makovai]

I lost the M/C on my '91 about 2 months ago to that very thing. Was only bleeding it after swapping in the 14bff.

So everything looked good, no leaks, cept you had no brakes?
And it was the M/c?

 

[ntsqd]

Everything except the fluid looked good. We made a point of flushing the system in the process of bleeding it. A week or so later it was nasty looking, and that truck mostly sits. Mostly sits even when the brakes do work, which they weren't doing very well at that time. R&Ring the m/c cured the problem.

 

[makovai]

Everything except the fluid looked good. We made a point of flushing the system in the process of bleeding it. A week or so later it was nasty looking, and that truck mostly sits. Mostly sits even when the brakes do work, which they weren't doing very well at that time. R&Ring the m/c cured the problem.

Well i'm kinda passed that point. I have now replaced everything cept the pvalve and i still have lousy brakes. They kinda work but if i was in a dangersous situation i'm in trouble, cause my stopping distance is horrid. And the absolutely will not lock up.

Hey maybe i have a poor mans ABS system