Hard time bleeding brakes

[Bert87]

So over Thanksgiving I rebuilt the calipers and replaced the soft lines on my 71 911T. Everything went pretty well but I am unable to properly bleed the brakes.

On disassembly I blocked the brake pedal just forward of engagement but forgot to plug the lines. So far I have tried regular old 2 person bleeding, gravity bleeding, and using the air compressor hooked to the vent line. Seemed to take a long time to fill the lines/calipers initially. I started with 2 person bleeding, then gravity, then pressure. Reservoir never ran empty.

On the last pass with the pressure I ran the majority of a reservoir through each caliper starting at drivers rear -> pass rear, pass front, drivers front. No bubbles seen.

I tried the brakes before I relieved the pressure and the pedal felt exactly as it should. Very firm about 1/2 way through its travel. Stayed firm while keeping pressure. Without the pressure though it doesn't start to firm up till its at the floor. I can stop but not lock up the tires.

I have been reading about new seals sucking the pistons back in farther than normal but I feel like it shouldn't be this drastic. Any suggestions for other bleeding methods?

[Rawknees'Turbo]

Maybe you've already done this, but when the calipers have been overhauled, you need to vigorously press the pedal from time to time (all caliper bleed nips closed) when doing the pressure bleed method . . . that helps set the pistons into position, as pressure bleeding alone will not do that.

I recently did all the calipers and hoses on an Infiniti - bled using the compressor pressure method, and it took a lot more fluid than you mentioned to get the pedal to feel right (with lots of vigorously pedal pumping along the way).

[Bert87]

Originally Posted by Rawknees'Turbo

Maybe you've already done this, but when the calipers have been overhauled, you need to vigorously press the pedal from time to time (all caliper bleed nips closed) when doing the pressure bleed method . . . that helps set the pistons into position, as pressure bleeding alone will not do that.

I recently did all the calipers and hoses on an Infiniti - bled using the compressor pressure method, and it took a lot more fluid than you mentioned to get the pedal to feel right (with lots of vigorously pedal pumping along the way).

I went through almost 2 liters in total.

I pumped the brake quite a bit with the bleeders closed but not while the pressure was applied (besides the time I tested before remove the air).

[Rawknees'Turbo]

I meant pumping the pedal without the air pressure applied, so it sounds like you are doing everything correctly. It sure seems like you must have a slug of air trapped someplace. Have you tried tapping on the calipers with a rubber mallet while pressure is applied and the bleed screw(s) is open? That can help free trapped air.

And just to make sure, are the calipers installed on the correct sides (with bleed nips facing up)? I ask, because people here with similar bleeding difficulties discovered that they had installed the calipers on the wrong sides (bleed nips facing down).

[Bert87]

Originally Posted by Rawknees'Turbo

I meant pumping the pedal without the air pressure applied, so it sounds like you are doing everything correctly. It sure seems like you must have a slug of air trapped someplace. Have you tried tapping on the calipers with a rubber mallet while pressure is applied and the bleed screw(s) is open? That can help free trapped air.

And just to make sure, are the calipers installed on the correct sides (with bleed nips facing up)? I ask, because people here with similar bleeding difficulties discovered that they had installed the calipers on the wrong sides (bleed nips facing down).

Ya I tapped all around the calipers while pressure bleeding.

I only did one caliper at a time but I have also double checked the bleeders are at the top.

Was thinking about running a liter straight through pressure bleeding with all the bleeders cracked at the same time.

[Jesse16]

I think the standard pressure bleeder method is likely doing the job. Guessing you're still getting piston pull back from the new caliper seals in the piston bore. I think you need to check and make sure that the pads are set tightly to the rotor initially. And that means really snug! That little bit of space takes a lot of pedal travel to accomplish initially. That's my experience.

[T77911S]

how were the brakes before you did the work
I usually just gravity bleed mine, sometimes do the 2 person. took the 930 brakes completely apart last year. after doing my brakes I will go back about a week later and bleed again.


how is the MC? could be bad.

what does this mean:
"I tried the brakes before I relieved the pressure "

[juanbenae]

have you tried banging on the calipers with a rubber mallet or the like to free bubbles in the calipers? I installed a set of rebuilt front calipers this weekend and banged and quick pumped to get it close. will need to get some heat into them this weekend and another bleeding.

post caliper rebuild ive always had a somewhat soft pedal until getting some heat into them prior to one last bleeding

[Bert87]

By "I tried the brakes before I relieved the pressure " I mean that I jumped the gun on testing the brakes after pressure bleeding and tested them before relieving the 5 psi or so that the system was under pressure.

I tapped the calipers with the handle of a rubber gripped hammer while pressure bleeding.

Would it be ok to ride the brake a little to get temps up? Not able to really get close to locking them up from 30-40 mph.

[juanbenae]

I would think if you simply rode them some doing no more than 25 MHP it be ok, safely.. I drove mine very briefly last weekend and probably could not got them to lock up completely, but I had enough brake to be safe during the exercise.

[Bert87]

If I took the pads out, pumped the pistons out pretty far, then pushed them back just far enough to slide the pads back in theoretically that would minimize the amount that the seals are pulling the pistons back. Guess that will be the next attempt.

[Rawknees'Turbo]

Originally Posted by Bert87

By "I tried the brakes before I relieved the pressure " I mean that I jumped the gun on testing the brakes after pressure bleeding and tested them before relieving the 5 psi or so that the system was under pressure. . . .

Five psi does not work for me (way too slow) - I use about 17psi all all vehicles (including the Porsche) . . maybe upping your air pressure will help.

[T77911S]

bad MC.

I have had calipers that needed to be rebuilt, in fact even leaked a little and still had great brakes. that was my 77s

did I say bad MC

[SteveDraz]

I has similar issues after a rebuild. It's tough to get all the nooks and crannies completely full. I drove the car and got everything hot and just kept driving. It took care of itself.

[T77911S]

I removed calipers and MC, blew air thru the lines to blow out old fluid and did not have this much trouble.

[Targalid]

T77911S said it: bad MC You likely need a new master cylinder. You said you pushed the pedal to the floor. That took the piston past its previous length of travel to the unswept area of the master cylinder where resides the corrosion that has accumulated over the years. This cuts the piston seal and then it leaks within the master cylinder when you apply foot pressure allowing brake fluid to pass.

[CaptRudolf]

This response solved my problem. Lots of talk about bleeding bleeding bleeding, but making sure the pads are tight (in my case after caliper rebuilds) seems to be often overlooked.

Originally Posted by Jesse16

I think the standard pressure bleeder method is likely doing the job. Guessing you're still getting piston pull back from the new caliper seals in the piston bore. I think you need to check and make sure that the pads are set tightly to the rotor initially. And that means really snug! That little bit of space takes a lot of pedal travel to accomplish initially. That's my experience.