My 95 has a typical non-ABS master cylinder consisting of two hydralic circuits.
When applying a light touch to the brakes at a stop (or when creaping forward), the brake pedal sometimes gives and goes farther to the floor than is normal. Coming off the pedal and reapplying firms up the pedal.
The pedal holds firm if one applys a hard stop and does not come off the brake.
Does this indicate that the master cylinder is failing?
I would think that a master cylinder failure would typically effect just one of the circuits and would express itself by a pulling to the side when braking.
Or will the pressure regulator valve mask the symptom by allowing one circuit to bleed over to the other circuit?
I think answering brake questions worries some folks as you're talking about a 'life and death' thing here. So with the over-riding proviso that you should seek professional advice on things like brakes, I'll offer my observations on brake problems I have encountered. My list is by no means complete or I suspect comprehensive compared to a mechanic who deals with brake-problems everyday,...so here are my limited comments:
IME, a pedal which 'pumps-up' can mean there is air in the system (brake-bleed req here) or there is too much clearance between the rear-shoes and their drum. On my car the rear drums have a selfadjuster which is supposed to keep lining-drum clearance correct,..but these for some reason often dont work all that well.
Other faults I've heard of include: spongey, cracked, ballooning, hydraulic hoses (between the metal brake-lines and the road-wheels)
This is a pleasing sign ( the pedal not sinking to the floor) but to test for by-passing seals, a more moderate pressure is applied which is continued for minutes. Any sinking to the floor is a sign the mastewr-cyl needs overhauling or depending on the condition of its bore, replaced.
The faults I've encountered and their causes are:
The MC leaking fluid down the front of the booster (which indicates a leaking rear-seal)- pedal sinks slowly to the floor, (bypassing seals due wear and/or a scored-bore) - causing brake drag (faulty residual pressure valves),..but I'm sure there are others.
No,...the dual system duplicates the fronts wheels and one rear-wheel. So if one system fails, you may not even be aware of it.
Cant answer that as I dont know what you mean.
I'd get a w/shop manual, and follow their 'fault symptoms and remedial action' section. If you think things are still not OK,..spend the bucks at the brake-shop!!
NB
The power-brakes booster can mask some MC/ brake faults. It can be checked by turning the engine off, pump the brakes 5 times (to deplete any vac-resevoir) the while holding the pedal down with your foot, start the engine. The pedal should sink slightly as the engine applies vacuum to the booster's diaphragm. This test IMO, should only be regarded as a basic test.
I once had a brake problem in a Ford V8 where the booster would lose action while I was creeping along in traffic (very disconcerting!). This turned out (after a long time of hit and miss diagnosing) to be a faulty PCV valve which was robbing the booster of vacuum 'top-up' because the valve was allowing too much crankcase ventilation at idle and the fact it was positioned on the same plastic intake-manifold access tree. This why I say an expert is needed in many cases with brake problems.
You say the pedal holds firm if you press firmly on the brake pedal and hold. If the master cylinder had internal leakage past the seals, which is how they "fail", then firmly pressing on the pedal would cause the pedal to sink gradually, so sounds like the master cylinder is OK. Try having the brakes bled properly. That would be the first step to removing air in the lines which can also give a soft pedal. Maybe have them bleed the brakes twice. Once they get it set up, the second time is fairly easy, and most of your cost is in labor, so the small amount of extra fluid is only around $5 for the second time. That way you're sure you've got fresh fluid throughout the system, and that it is completely free of any hidden air pockets. If you still have a problem, then you can investigate further, but I suspect that will cure it, and is a good general maintenance procedure in any event.
Yep - thats what I am doing. I know that there are pros here! Are you one?
Besides, I want to understand the physics so that I am not ripped off by one of those supposed pros! b-)
My camry has disk brakes all the way around ... so it aint that.
But won't you only get that symptom if the seals for both circuits have failed? So then you need a double failure in order for that test to work?!
I'll check that but if there is a leak - its so slow that the fluid level is undetectable.
Didn't know that. But there are two circuits that come out of the master cylinder. Its a standard simple master cylinder!
Unfortunately, the Camry factory service manual doesn't explain (or even show) the layout of the brake system. Based on my reading knowledge, generally, some cars use a left front/right rear and a right front/left rear circuit split. Others work as you explain.
Typically, each circuit is directed back to a regulator value that is somewhere else in the car. The regulator serves two functions. First, it splits the circuit pressure so that the rear wheels get less pressure than the front wheels. Additionally, the regulator monitors the pressure between the two circuits and if there is a difference, the brake malfunction light is activated. Whoops, I miss spoke - this vehicle doesnt have one of these. Instead:
In this camry, there is a load sensiing proportioning valve (and no regulator). It is in the rear and monitors the amount of weight in the car by measuring the amount displacement of the axle relative to the body. Based on the amount of displacement, different amounts of pressure are given to the rear. (e.g., in a lightly loaded vehicle - more braking occurs on the front).
I got the factory service manual but it doesnt have much in diagnostics. The closest thing is "low pedal or spongy pedal" which suggests servicing up to 6 different issues (including air in line or master cylinder). But it does not provide guidance on distinguishing the malfunctions.
The physics of how a MC works involves seals, springs and valves. Diagrams are needed to understand exactly how they function.
Yes.
So then you need a double failure in order for that
Yes,..my terminology sucks. I meant to indicate any slow downward travel after applying moderate pressure over minutes, is not good. The reason I said "moderate pressure" is because high pedal-pressure can make an unacceptabley worn cup seal, regain complete sealing as its lip is forced against the cyl bore.
I was refering to a dual safety circuit, not front vs rear.
Once again, that is refering to the 'front-rear' situation.
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