Bubbling Brake Master Cylinder - Alfa Romeo Bulletin Board & Forums
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post #1 of 26 (permalink) Old 08-22-2019, 08:21 AM Thread Starter
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Bubbling Brake Master Cylinder

Hello All,

1991 Spider Veloce.

I looked at the many, many threads on brake bleeding but could not find this mentioned.

I'm still not very happy about the braking in my '91. It just take too long for my liking. I am unable to lock up the tires no matter how hard I push on the brake pedal. If you have ever had that holy sh$t moment where you needed to stop fast and couldn't, you will understand my worries.

I replaced the booster last year, but no real improvement.

I have some time on my hands today, so I decided to flush some new fluid through the brake lines.

I use speed bleeders on the calipers.

When I bled the rears, on the up stroke of the pedal, I could see a few small bubbles entering the rear of the master cylinder reservoir.

But the fronts is a totally different story.

When bleeding the fronts, on the down stroke of the brake pedal a ton of air bubbles continuously bubble up in the rear of the reservoir. I mean a ton, looks like its boiling. I could do this for 10 strokes and no change.

Both the front L and R exhibited the same behavior.

Bad MC?

Thanks in advance for any help.

Sincerely,

Vin

1991 Spider Veloce - Red on Tan - Rosa Bionda
1987 Spider Quadrifoglio - Red on Grey - Rosa - Sold
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Last edited by Vintre; 08-22-2019 at 03:04 PM.
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post #2 of 26 (permalink) Old 08-22-2019, 08:53 AM
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perhaps the threads of the speed bleeders are allowing air back in when you loosen them to allow flow?....you might need some teflon tape on the threads.

Long pedal travel might be booster rod play, but I think you adjusted that in an earlier thread?

Dom - Alfa Spider 1990 S4 - formerly: Alfa 101 Sprint, 2600 Sprint, Montreal - family classics: Jensen Interceptor II, '58 Hooper RR Silver Cloud I, Shadow II, '60 Corvette.
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post #3 of 26 (permalink) Old 08-22-2019, 09:06 AM
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A lot of hanging pedal MCs have long travel; it seems an unfortunate design thing. Norman Racing has talked about trying to modify it out, without success.
If I was unsure of my MC and didn't know how old it was, I'd replace as long as you're bleeding in any case.
I use a bleeder hose with ball-type check valve at the end, which goes in the jar o' fluid. Same idea as speed bleeders but maybe not as messy. I haven't had issues.

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post #4 of 26 (permalink) Old 08-22-2019, 09:12 AM
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I don’t see how anything at the bleeders could get air all the way to the MC. Sounds like a problem with the MC itself.
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1963 Giulia Spider (1750 engine)
1974 GTV
1991 Spider
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post #5 of 26 (permalink) Old 08-22-2019, 09:15 AM Thread Starter
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Quote:
Originally Posted by spiderserie4 View Post
perhaps the threads of the speed bleeders are allowing air back in when you loosen them to allow flow?....you might need some teflon tape on the threads.

Long pedal travel might be booster rod play, but I think you adjusted that in an earlier thread?
I can put some teflon tape on the bleeders, easy enough.

Yes, I did adjust the push rod pin and believe its at the proper height.

Why so many bubbles from the front when pushing down on the pedal though? Its weird.

Vin

1991 Spider Veloce - Red on Tan - Rosa Bionda
1987 Spider Quadrifoglio - Red on Grey - Rosa - Sold
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post #6 of 26 (permalink) Old 08-22-2019, 09:19 AM
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Well, there is a different piston/seal for each circuit. The front seems to move more fluid on each stroke, so maybe related to that due to larger caliper pistons in front.
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post #7 of 26 (permalink) Old 08-22-2019, 10:04 AM
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Quote:
Originally Posted by Gubi View Post
I don’t see how anything at the bleeders could get air all the way to the MC. Sounds like a problem with the MC itself.
+1

Rich Hanning
'65 Sprint GT, '78 Spider, '88 Quad
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post #8 of 26 (permalink) Old 08-22-2019, 10:30 AM
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Quote:
Originally Posted by Vintre View Post
I can put some teflon tape on the bleeders, easy enough.

Yes, I did adjust the push rod pin and believe its at the proper height.

Why so many bubbles from the front when pushing down on the pedal though? Its weird.

Vin
Teflon tape is generally advised against in braking applications (though some people do it anyways). You don't want it breaking down from the fluid an springing leaks later.

There are basically three possibilities:

1) You have air in the MC itself that is being pushed out- bleeding insufficient
2) MC is bad, preventing air from getting out
3) You have a leak somewhere in the system

You might try bench bleeding the MC. And bleeding the whole system again. If that still doesn't work, I'd look at replacing the MC. I'd also look into pressure bleeding the system instead of using the speed bleeders.
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post #9 of 26 (permalink) Old 08-22-2019, 10:46 AM
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Vin, I suggest you first check for a frozen piston/pistons in the calipers. Make certain you're getting the clamping force you need. How old are the caliper seals, BTW?

Next, I agree also about the MC as a prime suspect. You may be actually pulling in air on the return stroke, which can never be 100% removed the way you're doing it. It can be entering via the pushrod lipseal, or maybe even the reservoir seals.

Last, get a pressure bleeder, like Motive Power. With 5 to 7 psi behind that brake fluid, it will chase the bubbles out, believe me! And most speed bleeders I've bought have thread sealant on them anyway, so they should be fine. I'm sure you know the caliper bleed order, RR, LR, RF, LF.
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post #10 of 26 (permalink) Old 08-22-2019, 11:31 AM Thread Starter
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Quote:
Originally Posted by alfaloco View Post
Vin, I suggest you first check for a frozen piston/pistons in the calipers. Make certain you're getting the clamping force you need. How old are the caliper seals, BTW?

Next, I agree also about the MC as a prime suspect. You may be actually pulling in air on the return stroke, which can never be 100% removed the way you're doing it. It can be entering via the pushrod lipseal, or maybe even the reservoir seals.

Last, get a pressure bleeder, like Motive Power. With 5 to 7 psi behind that brake fluid, it will chase the bubbles out, believe me! And most speed bleeders I've bought have thread sealant on them anyway, so they should be fine. I'm sure you know the caliper bleed order, RR, LR, RF, LF.
I rebuilt all 4 calipers last year. Seals are new.

Will try bench bleeding the MC and I do follow the caliper bleed order.

Thanks for all the help everyone.

Vin

1991 Spider Veloce - Red on Tan - Rosa Bionda
1987 Spider Quadrifoglio - Red on Grey - Rosa - Sold
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post #11 of 26 (permalink) Old 08-22-2019, 11:43 AM
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Hi there, when bleeding, after the pedal has hit the floor, that first return up from the floor I usually do that slowly so as not to draw air in down from the reservoir. Which anything less that a half reservoir seems possible. I'm not familiar with speed bleeders etc, I only ever do the two person method....

Cheers,

Carson, 4 Alfa's, 9 Cars, 4 Motorcycles
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post #12 of 26 (permalink) Old 08-22-2019, 11:44 AM
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An old hydraulics guy question... are all the hose and tubing fittings tight, from the MC out to the wheels?

I like to do progressive bleeding, when I bolt on new components and refill, starting at the MC, out to the fitting unions at the suspension, then to the hoses at the calipers, then the caliper bleed fittings. I'll just crack the fittings open until I get solid liquid, no bubbles. Then I move to the next fitting in line, and repeat. Works fine with a pressure bleeder, otherwise the bubbles like to rise to the highest point.
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post #13 of 26 (permalink) Old 08-22-2019, 12:19 PM
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I just don't see the MC needing bench bleeding, as Vin only wanted to flush the system thru with some new fluid....he is a competent enough guy not to have let the MC flow empty.
You only really need to bench bleed a new MC.

maybe a reverse pressure bleed might be in order......from the calipers back up to the MC

what I like to do is hit the pedal 3 or 4 times then hold it down, and THEN open the bleed nipple, and as the pedal hits the floor, keep it there till the nipple is closed. Repeat till no air comes out....this way fluid really squirts out taking air with it. Of course this is a 2 man job.

Dom - Alfa Spider 1990 S4 - formerly: Alfa 101 Sprint, 2600 Sprint, Montreal - family classics: Jensen Interceptor II, '58 Hooper RR Silver Cloud I, Shadow II, '60 Corvette.
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post #14 of 26 (permalink) Old 08-22-2019, 12:35 PM
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I just use my MightyVac hand pump bleeder. So easy, one person job.
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post #15 of 26 (permalink) Old 08-22-2019, 01:52 PM Thread Starter
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Update:

I removed the MC completely and bench bled it. No bubbles whatsoever.

Re installed on the car and bled the brakes in the correct order (even had the car idling while doing that. I read somewhere that someone said it would help. Can't hurt).

While bleeding the fronts those massive bubbles are now gone on the down push of the pedal and I dont see any bubbles anywhere on the release of the pedal.

While I now have a bench bled MC, bled brake lines and fresh fluid (used 1 litre), no improvement on the braking.

So what do you figure now? Change out the MC and do this all again?

Vin
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