Another brake question?

I posted about my Chevy 3500 yesterday, I forgot to mention the USPS owned the truck before I bought it, they had a sticker on the underside of the hood to use only dot 5 brake fluid, which I have until last night. Any idea why they would use that? At $31.00 a quart we just put dot 3 back in it.

Most likely a fleet truck and many of those are a special order type of thing and parts for some can be hard to come by

Like Old said. Fleet vehicles for the government can be special ordered with all sorts of options or even switched over after being put into use. They can run many different things in them from fluids to hardware to cut down on maintenance intervals.

Greg

If it was ordered with silicone from the factory , it may have different parts in the MC and WC's Silicone fluid will work with both types , but the oil base parts will swell and be ruined by Brake fluid. Might want to pull apart a wheel cyl and examine /test the cups for compatability...

DOT 5 does not absorb moisture from the atmosphere and will not damage paint. It is usually used in rarely driven collector cars with fancy paint that are prone to brake problems, i.e. Corvettes.

I don't believe you can mix dot 3 with dot 5.
Dot 5 is silicone base and Dot 3 glycol based.
Do a search for "mixing Dot 3 with dot 5 brake fluid"

(quoted from post at 07:52:57 12/05/15) I posted about my Chevy 3500 yesterday, I forgot to mention the USPS owned the truck before I bought it, they had a sticker on the underside of the hood to use only dot 5 brake fluid, which I have until last night. Any idea why they would use that? At $31.00 a quart we just put dot 3 back in it.

Click to expand...

You could google it are take your chance here on some wild arse guess...

http://www.mossmotors.com/SiteGraphics/Pages/Brake_Fluid/page4.html

I don't like it, it does not make the system water proof if water gets in the system and it will over time were that one drop of water sits will rust and ruin the part. I have dealt with some re-builders of brake calipers that state warranty is void if dot 5 is used for this very reason.

I have seen dot 3 and 5 mixed it did not cause a issue I know of. I did the maintenance on the trucks for the local USPS for 30 years till I ran that sorry bunch off dead beats off.... They were outfitted with dot 5 I have seen them mixed with no issues.

Either way you go you could find a shop that has flush equipment and have the system flushed its not money wasted. I have a set up that's decanted to both I don't mix.

If you topped off with a little dot 5 suck it out of the reservoir fill it with dot 5. I don't think the seals in the system care either way dot 3 are 5... The only issue I ever had was the cover seal on the master swell/deform but none with the master cyl calipers are wheel cylinders that were not related to age... YMMV..

Stop buy I will sell you a gal of dot 5 for $50 and be happy to shed myself of it I have 6 gal you can have it all...

BYW some may can read your mind are keep up with your history...
Year make and model would help us that can not... :wink:

DOT5 silicone brake fluid should not be mixed with DOT3, DOT4 or (interestingly) DOT5.1 fluid. If you switch from DOT5 to a glycol fluid like DOT3, you should thoroughly flush the brake system. Given that it sounds like it won't be long before you have to go completely through the brakes on your truck, I suppose it doesn't make much difference at this point.

I'm guessing the USPS went with DOT5 either to reduce maintenance (it doesn't absorb water) or because they thought it would perform better in the stop-start postal service driving.

I inadvertently put DOT4 fluid in the front brakes of my wife's Harley Sportster. (Harley uses DOT5 because it won't damage paint.) When I realized my mistake I flushed it out with DOT5. It was probably a coincidence that the master cylinder failed a year later.

Dot 3 should not go into a system with dot 5 in it . It may swell rubber parts and block bleed back holes.