<VV> Temporary loss of brakes when in reverse

[Frank DuVal]

This failure happens with dual cylinder master also! In fact, I've 
replaced more dual for this issue than singles! My very first experience 
with this failure was pulling up to a gas pump and apply brakes slightly 
as I was going slow in my neighbor's 1968 Valiant (yes, dual master 
cylinder). Lucky I missed the pump and the second press made the brakes 
work. Since the fluid bypasses the cups between the two circuits in the 
master, both circuits fail together! 😮

I quit doing dual master changeovers and just make sure the single pot 
systems are in GOOD shape. Do the hard Harder HARDER test at least 
anualy if not every time you get into the car.

That's step on the brake pedal. Step harder. Grab the steering wheel and 
STAND on the pedal. Oddly, this will not make the bypass failure happen 
but it is GREAT at finding all the other weak spots in a hydraulic 
system: hoses, steel lines, wheel cylinders, etc.

The test for bypass failure is step on pedal very lightly and follow the 
pedal to the floor if it fails.😉

Frank DuVal


On 8/24/2026 7:22 PM, eric.michelson at verizon.net wrote:
> Thank you to both Jay and Frank. This is probably going to be the push to upgrade the MC with a dual cylinder. I don't want to repeat the experience of losing the brakes.
>
> Eric
>
> -----Original Message-----
> From: VirtualVairs<virtualvairs-bounces at corvair.org> On Behalf Of Jay via VirtualVairs
> Sent: Monday, August 24, 2026 1:13 PM
> To: Frank DuVal<corvairduval at cox.net>
> Cc:virtualvairs at corvair.org;eric at michelsonshoes.com
> Subject: Re: <VV> Temporary loss of brakes when in reverse
>
> Greetings,
>      I have experienced perfect brakes, then all of a sudden the peddal goes to the floor!  Turned out, the mosture absorbed into the brake fluid,
> causes an accidic compound in the oil.   This eats away at the aluminum
> piston in the Master Cylinder.  When those little "holes" dinally break through - there is NO BRAKING!
>
>      Mine had rotted through in my '66 Corsa 140, and late one evening while heading home the pedal went for the floor.  I rolled through a four-way intersection that lukily had no other cars in it!  The trip home (35
> miles) was done using my Emergency Brake.  Tore down the Master and found that the piston was swiss cheese!  Got a rebuild kit ASAP.
>
>      If your peddal ever goes to the floor, best to replace the guts of the MC and be sure you, and others on the rode, as safe.
>
>         --- Jay Nugent
>             Ypsilanti, MI
>             '64 Greenbrier
>
>
> On Mon, 24 Aug 2026, Frank DuVal via VirtualVairs wrote:
>
>> When the brake pedal goes to the floor with a light touch of the foot but it works fine with harder application,  it means the master cylinder needs rebuild/replace.
>>
>> This is for both single port and dual port master cylinders.
>>
>> Frank DuVal.
>>
>>> On Aug 24, 2026, at 7:33 AM, eric--- via VirtualVairs<virtualvairs at corvair.org> wrote:
>>>
>>> I had stopped in a parking lot with my '64 coupe 95hp PG running errands.
>>> When I restarted the car and shifted into reverse the brake pedal
>>> went straight to the floor. I was able to shift into forward and not
>>> run over the pedestrian behind me. The brake pedal immediately
>>> regained pressure and the brakes worked fine on my ride home. I do
>>> have brakes in both forward and reverse.
>>>
>>> I have not seen any fluid leakage on the garage floor and have been
>>> too busy to get it on the lift. It's a low mileage car (19K) and the
>>> PO had changed the brake lines when he awoke it from a 40 slumber.
>>> Still has a single master cylinder and it seems to be in great
>>> condition, maybe a little low on fluid.
>>>
>>> Before I dig into this I'm looking for some clue as to why I had (for
>>> now) a one-time loss of pedal pressure. I have not driven it since
>>> getting it home that day.
>>>
>>>
>>>
>>> Eric
>>>