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Bubbling Brake Master Cylinder

4K views 25 replies 12 participants last post by  bianchi2 
#1 · (Edited)
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
 
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#2 ·
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?
 
#5 ·
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
 
#3 ·
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.

Andrew
 
#9 ·
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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#10 ·
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
 
#11 ·
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,
 
#22 ·
I just flushed the break fluid from my 84 a couple of weeks ago.
1. Two person job
2. Clear hole and jar
3. Fill reserve
4. Start at passenger rear wheel.
5. Have your assistant pump and hold
6. Open bleeder value.
7. Have assistant hold down the peddle.
8. Close bleeder value.
9. Repeat until you do not see any more bubbles.
10. Check reserve often and do not let reserve get 1/2 empty.
Work your way around the car driver rear. Passenger front and driver front. If you still do not have a stiff peddle, replace the master cylinder.

Good luck.

PS I have tried other methods and failed.


Sent from my iPad using Tapatalk
 
#12 ·
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.
 
#13 ·
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.
 
#14 ·
I just use my MightyVac hand pump bleeder. So easy, one person job.
 
#15 ·
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
 
#16 ·
Vin, if you are certain the booster is working, and you have adequate vacuum to it, then there is still air being compressed in the system. That's what I think at this point. Check the calipers and pads while someone pumps the brakes, see if they are actually moving and clamping the discs.
 
#17 ·
Is the pedal firm? If so, it’s not the air keeping you from getting good clamping force. If there’s only a little air, the pedal will just be a bit mushy.

If it IS firm but you still don’t have good brakes, it’s probably the booster or the vacuum to it.
 
#19 ·
Are your rotors new? If they are worn, new, slightly oversize pads could extend inward over the lip that is created around the inner circumference of the swept area. The end result is the full force of the brakes is only acting on the small, inner portion of the disk that has not been worn. This happened on the '88 earlier in the year. I installed new pads and had very poor brake performance. Then realized the new pads were slightly larger than the old ones.
 
#20 ·
While I have done a lot of work on this car, the rotors are the same ones as I when I purchased the car in May 2008.

I went out and felt the surface of all 4 rotors. I do not feel any ridges or lips.

While the LR is smooth and flat, the other 3 are smooth but have a waviness to them that I can feel. Nothing huge, but I can feel it.

Vin
 
#21 ·
That was my shot in the dark. I don't think it is unusual to feel or see some irregularities across the radius of the swept area. If the waviness is around the circumference of the rotor, I would expect a pulsating pedal when the brakes are applied.
 
#23 ·
I agree it sounds like your booster is working, but I don't like your description of those three discs, Vin. If they are glazed over, or have an uneven surface, you're not going to maximize the friction applied by the pads. It might be worth it to take them off, have them cleaned up on a brake lathe, just enough to rough up the surface again. BTW what kind of pads are you running?
 
#24 ·
Once, years ago, I had a 70's Spider in the shop that just didn't stop well. Firm pedal, good calipers and rotors, thick pads, etc. To shorten a long story, I replaced the pads and fixed the problem. I guess, sometimes the friction material goes bad. I have a thick piece of glass, cut so that a piece of 80 grit self-adhesive file board sand paper fits to, that I use to "resurface" old brake pads. Sometimes taking off a 32nd or two gets you down to a better layer. Sometimes even then, you just need to replace the pads.
 
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#26 ·
I always bled my brakes this way, with no problems.. long clear vinel hose, about 20-24 inch long, empty pastic empty water bottle.. pour about 1.5 ins of brake fuild into the bottle, put one end of hose into the bottle, crack open bleader, push hose oven it, make real shure that the hose rises about 8-10 inchs above the caliper that you are working on then into the water bottle.. with blead screw open about 1/4 to 1/2 turn,, pump brake pedal down slowly a few times,, keeping fuild level up in master cly. watch the clear hose for any bubbles.. then go to next caliper.. repeate for each caliper.. make shure that yo don't over run water bottle catch for brake fluild. I like to make 2 runs aroung the car, just to make shure.. you would be susprised when you watch the clear vinel hose, how many really tiny bubbles you will see.. this is one person job, just a wee bit of jogging around the car.. also remember to shed some blood to the alfa gods during repair.
 
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