1989 Subaru Justy
I start noticing my brakes not working as well as they should. When I pump the pedal a few times they get solid again for a few seconds.
I bleed the brand lines and it is good for about 6 months, then they start getting soft again.
I don’t see any wet spots and the reservoir is still full.
Should I assume I need a new master cylinder, or something I can test?
That’s what I would suspect.
Tester
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Could air be somehow leaking in to the brake system?
How would bleeding the brakes fix a bad master cylinder for a while?
Air can enter the brake system thru the piston seals on the calipers.
When this happens, when the brakes are applied the hydraulic pressure causes the seals to expand as they should.
But when the brakes are released, the action of the piston being drawn back into the caliper can allow a small amount of air to be sucked past the seal allowing air into the caliper causing a soft brake pedal.
Then when the brakes are bled you get a firm pedal.
But the more the brakes are used, more air gets drawn back into the caliper and the soft pedal returns in a short time.
Not six months.
Tester
OP, press the brake pedal firmly and hold for 2 minutes. Does the pedal slowly move toward the floor? If so, the only way that can happen is (1) there’s a brake fluid leak, which would be visible upon inspection under the car, or (2) the brake master cylinder is failing, internal seal leak. I don’t know if pedal could move to floor without visible leak w/a failing ABS unit, but presuming your 89 Justy doesn’t have the ABS function.
If brakes were bled using the push on the brake pedal method prior to noticing this symptom, good chance the bleeding process damaged a MC internal seal. During that type of brake bleed procedure the MC piston moves into MC bore areas not reached during normal braking, and if there’s a spur, corrosion, gunk etc on inside bore, that movement will damage the seal.
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I think that, after 33 years, you should assume that the master cylinder needs to be replaced, Actually, I wouldn’t be surprised if this is the 2nd or 3rd time that this old car has needed to have its master cylinder replaced.
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I drove a lot of old cars ( I commuted for years with a 66 Valiant starting in 1989.)
I only replaced one master cylinder in 70 years of driving.
That’s a pretty impressive record there OT-11. 30 year old Corolla brake system retains original MC, but 50 year old Ford truck is on MC number 5 or 6.
Never replaced, but the VW Rabbit’s MC had to be rebuilt nearly every year. Sort of an annual event …lol …
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Well, I never owned a Ford or an MG. I owned 2 Studebakers, too many Chrysler products to list, my current 2012 Camry and a 72 Impala and a 98 Olds Intrigue. The last two were the worst cars I ever had.
I had a friend with a K car (either Aires or Reliant) who had a rear brake job done at the dealer. He had to keep returning to the dealer because of the brake pedal getting lower and lower. They would adjust the brakes and send him on his way.
It was driving him nuts because it was a company car and he had to drive it every day to estimate construction bids. I told him I would look at it.
I pulled both rear drums and found a new left hand brake adjuster on the right side of the car. It is not unusual for these to rust solid in our salt encrusted area. I am guessing the dealer did not have the right side one in stock so the tech just stuck a left side one in it’s place.
Every time he backed uo and stepped on the brake, it unadjusted the right side.
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I bled the brakes yesterday and the front two kept spitting out an occasional air bubble. I ran a half cup of fluid through each side.
The back brakes were fine. The front pads are looking thin so I’ll replace those when I do the master cylinder.
I’m thinking air in brake lines isn’t the cause of occasional soft pedal. I had this same problem recently on my truck’s drum brakes, caused by broken brake spring. Your Justy front brake probably disc with caliper design. A broken spring inside a caliper-bore could cause that. Or if the caliper wasn’t sliding freely on the slides, that might be a cause too I think. Since you will be replacing front pads, suggest to inspect for those problems too.
Note: If you have ABS, be sure to open bleeder screw before retracting pads, otherwise risk contaminating ABS unit.