Brake issues..., New Master Cylinder but... Options

[streetrover]

I installed a new Master Cylinder yesterday. Adjusted the rear calipers and bled all the lines. Rears work great but fronts don't squeeze? Any thoughts?

Thanx,
Bob (IMG:style_emoticons/default/confused24.gif)

[rnellums]

how old are the lines?

[streetrover]

Rebuilt all four calipers. Brand new flexible lines at all corners. Hard lines original to car ('75). Strangely, I can vacuum bleed fronts...so fluid gets through. Just no squeeze in the front.

Bob

[VaccaRabite]

Is the pedal firm or mushy? My guess is that you have more bleeding to do. Maybe you bled the lower valves on the front calipers and not upper ones?

[jk76.914]

I installed a new Master Cylinder yesterday. Adjusted the rear calipers and bled all the lines. Rears work great but fronts don't squeeze? Any thoughts?

Thanx,
Bob (IMG:style_emoticons/default/confused24.gif)

Same thing happened to me just last week. I replaced my original '76 front calipers with 911 M calipers, along with new flexible hoses, but keeping original hard lines and master cylinder. The new calipers were restored by Eric Shea, so I ruled out any problem with them immediately. I had no problem pressure bleeding- fluid came right through. And there was some movement of the calipers, just not enough to press the pads hard onto the rotor. Fixed it on Friday night. Here's the thread-

http://www.914world.com/bbs2/index.php?sho...p;#entry1513354

(Sorry if I get a bit verbose sometimes. I was excited about what I learnt!)

Jim

[streetrover]

OK, took a look at the thread you listed. Don't think that's gonna do the trick though. I can wiggle the pads about a quarter-inch on either side of the rotors. Pistons just don't wanna come out. I don't think its a problem with the calipers though because I rebuilt the rears and they're working and it seems a bit much that BOTH fronts would be stuck after the rebuild.

When I depress the pedal the rears squeeze the rotors nice and tight. Additionally, the pedal is nice and stiff (pedal goes in about an inch - master cylinder shaft about half an inch).

As I understand it there are two cups/seals inside the master cylinder, one creates pressure for the front brakes and the other creates pressure to the proportioning valve and then to the rear brakes. Is it possible that the pressure in the rear circuit locks the rear brakes (and stops the pedal from depressing further) BEFORE pressure is created in the front circuit?

I didn't bench-bleed the master before I installed it. Perhaps I should try that.

Please keep the ideas coming until the problem is solved! (IMG:style_emoticons/default/blink.gif)

Thankx,
Bob

[jsaum]

Here's a dumb question, because I rebuilt my calipers and couldn't get the air out of the system until I notice I mixed up my fronts and had the bleeder valve on the bottom and not at the top! Are they on the correct sides with the bleeders on the top?

Jsaum

[Mueller]

I've seen bad brand new master cylinders before, would suck if that is the case, but it happens

[914Mels]

Here's a dumb question, because I rebuilt my calipers and couldn't get the air out of the system until I notice I mixed up my fronts and had the bleeder valve on the bottom and not at the top! Are they on the correct sides with the bleeders on the top?

Jsaum

Bleed valves have to be to the top, you'll never get all the air out if they are on the bottom.

[Valy]

I installed a new Master Cylinder yesterday. Adjusted the rear calipers and bled all the lines. Rears work great but fronts don't squeeze? Any thoughts?

Thanx,
Bob (IMG:style_emoticons/default/confused24.gif)

1. Try to bleed the front calipers.
- If they bleed, rebuild the calipers.
- If you see no fluid coming out when you press the pedal it means the MC doesn't create pressure in the front circuit. Continue to step 2.
2. Crack the front lines connection to the MC and press the pedal.
- If it bleeds, your lines are blocked.
- if not, the MC is RIP or something is connected wrong.

Let us know where you get with this and will pick-up from there.

[streetrover]

Gonna try bleeding at Master Cylinder connections this afternoon. Will post afterwards.

Bob

[streetrover]

I've seen bad brand new master cylinders before, would suck if that is the case, but it happens

Yup...guess what...defective MC right out of the box.

Bled lines at master...no pressure at fitting. Disassembled new MC to find that front plunger and seals were stuck and the spring wouldn't push them back to original pedal position. Applied compressed air to push plunger back to discover that cup/seal was installed backwards (I think). Reversed the seal, reassembled the MC, vacuum bled lines, pushed pedal down about 2" nearly 500 times to squeeze front calipers together without pushing pedal to floor, then had daughter hold pedal down and bled brakes manually. Lastly I readjusted rears and now, finally, I've got a nice stiff pedal and all four corners are squeezing tightly. Whew!

Bob (IMG:style_emoticons/default/laugh.gif)