DOT 4 fluid for clutch- why? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

4793 · 09-07-2023, 09:40 PM

My avatar truck is an '02 K2500 with the 8.1/ 6spd manual. Noticed the other day the clutch reservoir was on the low side and *then* I noticed the cap said "DOT 4 only". I just had DOT 3 brake fluid on hand so I did a little research and made a phone call to a trusted mechanic- long story short I ended up going to town for a bottle of DOT 4. The brake system in this truck takes DOT 3 and I end up wondering why the clutch wouldn't use the same fluid?

(if this has been covered somewhere, apologies, the search results were inconclusive)

**hatzie** · 09-10-2023, 05:06 PM

DOT3 & DOT4 are both glycol based brake fluids. DOT4 has a significantly higher dry and wet boiling point. I'm not sure why they would use it in the clutch and not in the brakes from the factory. Good possibility someone changed the clutch master cylinder reserviour cap.

DOT4 is downward compatible so you don't have to clean the system entirely to update to DOT4. I run DOT4 in everything. An extra couple hundred degrees boiling point is a good thing when towing.

I changed the brakes on my 05 Silverado and my 09 Impala over to DOT4 when I flushed the brakes several years ago. I replaced the DOT3 master cylinder cap with the 92457850 DOT4 MC cap.
This is the post on flushing the brakes in my build thread. http://67-72chevytrucks.com/vboard/s...8&postcount=68

4793 · 09-11-2023, 09:45 AM

hatzie: thank you. It never occurred to me the cap might have been swapped out. Also thanks for explaining the difference between 3 and 4. My local guy said the same thing you do, that 4 could be added to 3 but not vice versa, but he wasn't real keen on the idea. The truck spent the first ten years of its life in the southwest US (not sure precise location) and maybe that has something to do with using a fluid with a higher boiling point

95 S_Trucker · 09-11-2023, 01:52 PM

The service manual calls for GM# 88958860

That is DOT 4 fluid. Nobody switched the cap.

**hatzie** · 09-11-2023, 08:10 PM

Quote:

Originally Posted by 95 S_Trucker

The service manual calls for GM# 88958860

That is DOT 4 fluid. Nobody switched the cap.

I've never had a T800 with a manual transmission.
Not sure what the reasoning was to do that. The clutch on my T400 used DOT3.

As I said. I run DOT4 in everything with brake fluid in it.

4793 · 09-13-2023, 12:29 PM

You're both right, it seems. The owner's manual refers to "GM Hydraulic Clutch Fluid" (which until now I didn't know was DOT-4) with the part number 95 S Trucker listed for the 6 speed, and DOT-3 for the 5 speed. I guess if they had to make things this complicated it's no wonder people gave up on sticks.

09-07-2023, 09:40 PM	#1
4793 Registered User Join Date: Dec 2022 Location: rural Iowa Posts: 20	DOT 4 fluid for clutch- why? My avatar truck is an '02 K2500 with the 8.1/ 6spd manual. Noticed the other day the clutch reservoir was on the low side and then I noticed the cap said "DOT 4 only". I just had DOT 3 brake fluid on hand so I did a little research and made a phone call to a trusted mechanic- long story short I ended up going to town for a bottle of DOT 4. The brake system in this truck takes DOT 3 and I end up wondering why the clutch wouldn't use the same fluid? (if this has been covered somewhere, apologies, the search results were inconclusive)

09-10-2023, 05:06 PM	#2
hatzie Moderator Join Date: Oct 2008 Location: Wentworth, NH Posts: 4,975	Re: DOT 4 fluid for clutch- why? DOT3 & DOT4 are both glycol based brake fluids. DOT4 has a significantly higher dry and wet boiling point. I'm not sure why they would use it in the clutch and not in the brakes from the factory. Good possibility someone changed the clutch master cylinder reserviour cap. DOT4 is downward compatible so you don't have to clean the system entirely to update to DOT4. I run DOT4 in everything. An extra couple hundred degrees boiling point is a good thing when towing. I changed the brakes on my 05 Silverado and my 09 Impala over to DOT4 when I flushed the brakes several years ago. I replaced the DOT3 master cylinder cap with the 92457850 DOT4 MC cap. This is the post on flushing the brakes in my build thread. http://67-72chevytrucks.com/vboard/s...8&postcount=68 __________________ 1959 M35A2 LDT465-1D SOLD 1967 Dodge W200 B383, NP420/NP201 SOLD 1969 Dodge Polara 500 B383, A833 SOLD 1972 Ford F250 FE390, NP435/NP205 SOLD 1976 Chevy K20, 6.5L, NV4500/NP208 SOLD 1986 M1008 CUCV SOLD 2000 GMC C2500, TD6.5L, NV4500 2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263 2009 Impala SS LS4 V8 RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials... 1973-1991 GM Light Truck Service, Unit/Overhaul, Wiring Manuals, Parts Books and Some 1968-1999 GM Service Training textbooks in PDF 1988-1998 GMT400 CK service manual project 1988-1993 GMT400 CK parts books. 1967-1972 GM Light, Medium, & Heavy Truck Service Manuals in PDF Please include at least the year and model in your threads. It'll be easier to answer your questions. And please let us know if and how your repairs were successful. Last edited by hatzie; 09-10-2023 at 05:17 PM.

09-11-2023, 09:45 AM	#3
4793 Registered User Join Date: Dec 2022 Location: rural Iowa Posts: 20	Re: DOT 4 fluid for clutch- why? hatzie: thank you. It never occurred to me the cap might have been swapped out. Also thanks for explaining the difference between 3 and 4. My local guy said the same thing you do, that 4 could be added to 3 but not vice versa, but he wasn't real keen on the idea. The truck spent the first ten years of its life in the southwest US (not sure precise location) and maybe that has something to do with using a fluid with a higher boiling point

09-11-2023, 01:52 PM	#4
95 S_Trucker Registered User Join Date: Oct 2009 Location: . Posts: 2,187	Re: DOT 4 fluid for clutch- why? The service manual calls for GM# 88958860 That is DOT 4 fluid. Nobody switched the cap.

09-13-2023, 12:29 PM	#6
4793 Registered User Join Date: Dec 2022 Location: rural Iowa Posts: 20	Re: DOT 4 fluid for clutch- why? You're both right, it seems. The owner's manual refers to "GM Hydraulic Clutch Fluid" (which until now I didn't know was DOT-4) with the part number 95 S Trucker listed for the 6 speed, and DOT-3 for the 5 speed. I guess if they had to make things this complicated it's no wonder people gave up on sticks.



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