The car had been sitting for awhile, and now I'm getting it running again.

The brake fluid reservoir is near empty... just a bit left at the bottom.
Yes, I know I need to bleed the brakes.
But for the moment, can I top off with DOT 3 without causing any damage?

Yeah, there'll be air in the lines, but that should come out wit a bleed.
This is only a temporary fix while I get the car functional again.

TIA

 

Yes. Do so immediately.

 

Welcome to the 1964Daimler,

I've moved your question from General Tech Help to MKI / MKII S type 240 340 & Daimler forum. This is the place to post tech questions about your model.

Graham

 

If the reservoir is nearly empty and there are no leaks on the floor, all the fluid has gone into the main vacuum cylinder of the brake booster in the fender well.
There is a seal at the end of the bore where the rod that operates the piston slides through _ this seal fails and dumps all the fluid into the cylinder.
In severe cases the vacuum of the engine can suck up the fluid and you will get large plumes of smoke when the engine burns the brake fluid.

At this point even if you don't have any air in the lines, I'm betting that all the calipers are in rough shape and may be rusty.
They may be seized and when you apply the brakes you will have uneven braking and a mushy pedal _ the car is not safe to drive.
It depends where it was sitting, how much moisture was in the air and if the area was heated.
Hygroscopic brake fluid is very nasty for old cars that sit _ in the best of conditions.

And DOT 3 is fine.

 

Yeah, that's what I'm afraid of: moisture in the brake system.
I don't know...
I'm hoping and praying....

How can one determine if the calipers and seals are okay?
Take off the wheels and visual inspection?
Or is there an instrument that can assist in making the determination?

 

Top up the fluid and start the car, if there is no smoke from burning brake fluid, drive the car out of the drive way and apply the brakes _ how does the pedal feel ?
If it's solid and doesn't travel excessively _ drive it around the block _ brake again _ how does feel ?

If these tests pass, the only way is to expel water and find out if there is water in the system is to bleed new fluid through.

The old stuff will be yellow, to being quite dirty _ when bleeding with new fluid on each caliper, keep going until the new clean fluid comes out the bleed nipple(s).
You don't have to drain the system though, as so may people do.

And it's not so much the water in the system, it's the damage it does. The bores in the calipers become rusty _ the rust expands, seizing the piston.
The brakes can feel firm, but they may stick on _ the drive around the block will tell you if one or more caliper is sticking _ you will also smell the pads getting hot.

It's common sense when testing out breaks.

The cylinders can be quite pitted from rust and they won't leak, they have to be excessively pitted before they leak. There is no way of knowing for sure unless you take one apart.

Honestly though, if no one has ever serviced the calipers and it has been sitting for along time, I can almost guarantee you that there is some pitting and rust in there _ but you may be lucky. Like I said before, it depends on how it was stored.