<?xml version="1.0" encoding="us-ascii"?><InstanceReport xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"><Version>2.4.0.8</Version><ReportLongName>060600 - Disclosure - Warranty Reserves</ReportLongName><DisplayLabelColumn>true</DisplayLabelColumn><ShowElementNames>false</ShowElementNames><RoundingOption /><HasEmbeddedReports>false</HasEmbeddedReports><Columns><Column FlagID="0"><Id>1</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><CurrencyCode /><FootnoteIndexer /><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios><MCU><KeyName /><CurrencySymbol /><contextRef><ContextID>c20130501to20130731</ContextID><EntitySchema>http://www.sec.gov/CIK</EntitySchema><EntityValue>0000354813</EntityValue><PeriodDisplayName /><PeriodType>duration</PeriodType><PeriodStartDate>2013-05-01T00:00:00</PeriodStartDate><PeriodEndDate>2013-07-31T00:00:00</PeriodEndDate><Segments /><Scenarios /></contextRef><UPS /><CurrencyCode /><OriginalCurrencyCode /></MCU><CurrencySymbol /><Labels><Label Key="CalendarSupplement" Id="0" Label="3 Months Ended" /><Label Key="Calendar" Id="1" Label="Jul. 31, 2013" /></Labels></Column></Columns><Rows><Row FlagID="0"><Id>1</Id><IsAbstractGroupTitle>true</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><Level>1</Level><ElementName>us-gaap_ProductWarrantiesDisclosuresAbstract</ElementName><ElementPrefix>us-gaap_</ElementPrefix><IsBaseElement>true</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>xbrli:stringItemType</ElementDataType><SimpleDataType>string</SimpleDataType><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>Warranty Reserves [Abstract]</Label></Row><Row FlagID="0"><Id>2</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><Level>2</Level><ElementName>us-gaap_ProductWarrantyDisclosureTextBlock</ElementName><ElementPrefix>us-gaap_</ElementPrefix><IsBaseElement>true</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><PreferredLabelRole>verboseLabel</PreferredLabelRole><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="c20130501to20130731" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText>&lt;div style="font-family: 'Times New Roman', Times, serif; font-size: 10pt;"&gt;&lt;div style="text-align: left; text-indent: 0pt; display: block; font-family: Times New Roman; margin-left: 0pt; font-size: 10pt; font-weight: bold; margin-right: 0pt;"&gt;Warranty Reserves&lt;/div&gt;&lt;div style="text-indent: 0pt; display: block;"&gt;&lt;br /&gt;&lt;/div&gt;&lt;div style="text-align: left; text-indent: 0pt; display: block; font-family: Times New Roman; margin-left: 0pt; font-size: 10pt; margin-right: 0pt;"&gt;Estimated warranty costs are accrued as revenues are recognized.&amp;#160;Included in the warranty cost accruals are costs for basic warranties on products sold. A summary of product warranty reserve activity for the three months ended July 31, 2013 is as follows:&lt;/div&gt;&lt;div style="text-indent: 0pt; display: block;"&gt;&lt;br /&gt;&lt;/div&gt;&lt;div style="text-align: left;"&gt;&lt;table cellpadding="0" cellspacing="0" style="width: 100%; font-family: times new roman; font-size: 10pt;"&gt;&lt;tr&gt;&lt;td align="left" valign="bottom"&gt;&lt;div style="text-align: left; text-indent: 0pt; display: block; font-family: times new roman; margin-left: 0pt; font-size: 10pt; margin-right: 0pt;"&gt;(Amounts in thousands)&lt;/div&gt;&lt;/td&gt;&lt;td align="left" valign="bottom" style="display: inline; font-family: times new roman; font-size: 10pt;"&gt;&amp;#160; &lt;/td&gt;&lt;td align="left" colspan="2" valign="bottom" style="display: inline; font-family: times new roman; font-size: 10pt;"&gt;&amp;#160;&lt;/td&gt;&lt;td align="left" valign="bottom" style="display: inline; font-family: times new roman; font-size: 10pt;"&gt;&amp;#160; &lt;/td&gt;&lt;/tr&gt;&lt;tr bgcolor="#cceeff"&gt;&lt;td align="left" valign="bottom" style="width: 88%;"&gt;&lt;div style="text-align: left; text-indent: 0pt; display: block; font-family: times new roman; margin-left: 0pt; font-size: 10pt; margin-right: 0pt;"&gt;Balance at May 1, 2013&lt;/div&gt;&lt;/td&gt;&lt;td align="left" valign="bottom" style="width: 1%; display: inline; font-family: times new roman; font-size: 10pt;"&gt;&amp;#160; &lt;/td&gt;&lt;td align="left" valign="bottom" style="width: 1%;"&gt;&lt;div style="text-align: left; text-indent: 0pt; display: block; font-family: times new roman; margin-left: 0pt; font-size: 10pt; margin-right: 0pt;"&gt;$&lt;/div&gt;&lt;/td&gt;&lt;td align="right" valign="bottom" style="width: 9%;"&gt;&lt;div style="text-align: right; text-indent: 0pt; display: block; font-family: times new roman; margin-left: 0pt; font-size: 10pt; margin-right: 0pt;"&gt;139&lt;/div&gt;&lt;/td&gt;&lt;td align="left" valign="bottom" style="width: 1%; display: inline; font-family: times new roman; font-size: 10pt;"&gt;&amp;#160; &lt;/td&gt;&lt;/tr&gt;&lt;tr bgcolor="white"&gt;&lt;td align="left" valign="bottom" style="width: 88%;"&gt;&lt;div style="text-align: left; text-indent: 0pt; display: block; font-family: times new roman; margin-left: 0pt; font-size: 10pt; margin-right: 0pt;"&gt;Additional reserves&lt;/div&gt;&lt;/td&gt;&lt;td align="left" valign="bottom" style="width: 1%; display: inline; font-family: times new roman; font-size: 10pt;"&gt;&amp;#160; &lt;/td&gt;&lt;td align="left" valign="bottom" style="width: 1%; display: inline; font-family: times new roman; font-size: 10pt;"&gt;&amp;#160; &lt;/td&gt;&lt;td align="right" valign="bottom" style="width: 9%;"&gt;&lt;div style="text-align: right; text-indent: 0pt; display: block; font-family: times new roman; margin-left: 0pt; font-size: 10pt; margin-right: 0pt;"&gt;180&lt;/div&gt;&lt;/td&gt;&lt;td align="left" valign="bottom" style="width: 1%; display: inline; font-family: times new roman; font-size: 10pt;"&gt;&amp;#160; &lt;/td&gt;&lt;/tr&gt;&lt;tr bgcolor="#cceeff"&gt;&lt;td align="left" valign="bottom" style="width: 88%;"&gt;&lt;div style="text-align: left; text-indent: 0pt; display: block; font-family: times new roman; margin-left: 0pt; font-size: 10pt; margin-right: 0pt;"&gt;Warranty reserve expense adjustments&lt;/div&gt;&lt;/td&gt;&lt;td align="left" valign="bottom" style="width: 1%; display: inline; font-family: times new roman; font-size: 10pt;"&gt;&amp;#160; &lt;/td&gt;&lt;td align="left" valign="bottom" style="width: 1%; display: inline; font-family: times new roman; font-size: 10pt;"&gt;&amp;#160; &lt;/td&gt;&lt;td align="right" valign="bottom" style="width: 9%;"&gt;&lt;div style="text-align: right; text-indent: 0pt; display: block; font-family: times new roman; margin-left: 0pt; font-size: 10pt; margin-right: 0pt;"&gt;(9&lt;/div&gt;&lt;/td&gt;&lt;td align="left" valign="bottom" style="width: 1%;"&gt;&lt;div style="text-align: left; text-indent: 0pt; display: block; font-family: times new roman; margin-left: 0pt; font-size: 10pt; margin-right: 0pt;"&gt;)&lt;/div&gt;&lt;/td&gt;&lt;/tr&gt;&lt;tr bgcolor="white"&gt;&lt;td align="left" valign="bottom" style="padding-bottom: 2px; width: 88%;"&gt;&lt;div style="text-align: left; text-indent: 0pt; display: block; font-family: times new roman; margin-left: 0pt; font-size: 10pt; margin-right: 0pt;"&gt;Charges incurred&lt;/div&gt;&lt;/td&gt;&lt;td align="left" valign="bottom" style="padding-bottom: 2px; width: 1%; font-family: times new roman; font-size: 10pt;"&gt;&amp;#160; &lt;/td&gt;&lt;td align="left" valign="bottom" style="border-bottom: black 2px solid; width: 1%; font-family: times new roman; font-size: 10pt;"&gt;&amp;#160; &lt;/td&gt;&lt;td align="right" valign="bottom" style="border-bottom: black 2px solid; width: 9%;"&gt;&lt;div style="text-align: right; text-indent: 0pt; display: block; font-family: times new roman; margin-left: 0pt; font-size: 10pt; margin-right: 0pt;"&gt;(54&lt;/div&gt;&lt;/td&gt;&lt;td align="left" valign="bottom" style="padding-bottom: 2px; width: 1%;"&gt;&lt;div style="text-align: left; text-indent: 0pt; display: block; font-family: times new roman; margin-left: 0pt; font-size: 10pt; margin-right: 0pt;"&gt;)&lt;/div&gt;&lt;/td&gt;&lt;/tr&gt;&lt;tr bgcolor="#cceeff"&gt;&lt;td align="left" valign="bottom" style="padding-bottom: 4px; width: 88%;"&gt;&lt;div style="text-align: left; text-indent: 0pt; display: block; font-family: times new roman; margin-left: 0pt; font-size: 10pt; margin-right: 0pt;"&gt;Balance at July 31, 2013&lt;/div&gt;&lt;/td&gt;&lt;td align="left" valign="bottom" style="padding-bottom: 4px; width: 1%; font-family: times new roman; font-size: 10pt;"&gt;&amp;#160; &lt;/td&gt;&lt;td align="left" valign="bottom" style="border-bottom: black 4px double; width: 1%;"&gt;&lt;div style="text-align: left; text-indent: 0pt; display: block; font-family: times new roman; margin-left: 0pt; font-size: 10pt; margin-right: 0pt;"&gt;$&lt;/div&gt;&lt;/td&gt;&lt;td align="right" valign="bottom" style="border-bottom: black 4px double; width: 9%;"&gt;&lt;div style="text-align: right; text-indent: 0pt; display: block; font-family: times new roman; margin-left: 0pt; font-size: 10pt; margin-right: 0pt;"&gt;256&lt;/div&gt;&lt;/td&gt;&lt;td align="left" valign="bottom" style="padding-bottom: 4px; width: 1%; font-family: times new roman; font-size: 10pt;"&gt;&amp;#160; &lt;/td&gt;&lt;/tr&gt;&lt;/table&gt;&lt;/div&gt;&lt;div style="text-align: left; text-indent: 0pt; display: block; font-family: Times New Roman; margin-left: 0pt; font-size: 10pt; margin-right: 0pt;"&gt;&amp;#160;&lt;/div&gt;&lt;/div&gt;</NonNumbericText><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>nonnum:textBlockItemType</ElementDataType><SimpleDataType>na</SimpleDataType><ElementDefenition>The entire disclosure for standard and extended product warranties and other product guarantee contracts, including a tabular reconciliation of the changes in the guarantor's aggregate product warranty liability for the reporting period.</ElementDefenition><ElementReferences>Reference 1: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 460

 -SubTopic 10

 -Section 50

 -Paragraph 8

 -URI http://asc.fasb.org/extlink&amp;oid=6398077&amp;loc=d3e12565-110249



Reference 2: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name FASB Interpretation (FIN)

 -Number 45

 -Paragraph 14

 -Subparagraph a, b

 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.



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